feat: add WCP approval, plan rollback, and webhook CRUD methods

Add approveStep, rejectStep, getPendingApprovals for WCP step gating.
Add rollbackPlan for plan version rollback. Add webhook subscription
CRUD (createWebhook, getWebhook, updateWebhook, deleteWebhook, listWebhooks).
New types: RollbackPlanResponse, VersionConflictException, webhook types.
All with comprehensive tests.

Author: saurabhjain1592

src/main/java/com/getaxonflow/sdk/AxonFlow.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2025 AxonFlow
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.getaxonflow.sdk.exceptions;
+
+/**
+ * Thrown when a plan update fails due to a version conflict (HTTP 409).
+ *
+ * <p>This indicates that the plan was modified by another client between
+ * the time it was read and the time the update was attempted. The caller
+ * should re-read the plan, resolve any conflicts, and retry with the
+ * updated version number.
+ *
+ * <p>Example usage:
+ * <pre>{@code
+ * try {
+ *     axonflow.updatePlan(planId, request);
+ * } catch (VersionConflictException e) {
+ *     System.out.println("Conflict on plan: " + e.getPlanId());
+ *     System.out.println("Expected version: " + e.getExpectedVersion());
+ *     System.out.println("Current version: " + e.getCurrentVersion());
+ *     // Re-read and retry
+ * }
+ * }</pre>
+ */
+public class VersionConflictException extends AxonFlowException {
+
+    private static final long serialVersionUID = 1L;
+
+    private final String planId;
+    private final int expectedVersion;
+    private final Integer currentVersion;
+
+    /**
+     * Creates a new VersionConflictException.
+     *
+     * @param message         the error message
+     * @param planId          the plan that had the conflict
+     * @param expectedVersion the version the client expected
+     * @param currentVersion  the actual current version on the server, or null if unknown
+     */
+    public VersionConflictException(String message, String planId, int expectedVersion, Integer currentVersion) {
+        super(message, 409, "VERSION_CONFLICT");
+        this.planId = planId;
+        this.expectedVersion = expectedVersion;
+        this.currentVersion = currentVersion;
+    }
+
+    /**
+     * Returns the plan ID that had the version conflict.
+     *
+     * @return the plan ID
+     */
+    public String getPlanId() {
+        return planId;
+    }
+
+    /**
+     * Returns the version the client expected.
+     *
+     * @return the expected version number
+     */
+    public int getExpectedVersion() {
+        return expectedVersion;
+    }
+
+    /**
+     * Returns the actual current version on the server.
+     *
+     * @return the current version, or null if unknown
+     */
+    public Integer getCurrentVersion() {
+        return currentVersion;
+    }
+}

src/main/java/com/getaxonflow/sdk/exceptions/VersionConflictException.java

@@ -30,7 +30,8 @@
  * ├── ConnectionException       - Network/connection errors
  * ├── ConfigurationException    - Invalid configuration
  * ├── ConnectorException        - MCP connector errors
- * └── PlanExecutionException    - Plan generation/execution errors
+ * ├── PlanExecutionException    - Plan generation/execution errors
+ * └── VersionConflictException  - Optimistic concurrency version conflict
  * </pre>
  *
  * <h2>Usage Example</h2>

src/main/java/com/getaxonflow/sdk/exceptions/package-info.java

@@ -0,0 +1,97 @@
+/*
+ * Copyright 2025 AxonFlow
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.getaxonflow.sdk.types;
+
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+import java.util.Objects;
+
+/**
+ * Response from cancelling a multi-agent plan.
+ */
+@JsonIgnoreProperties(ignoreUnknown = true)
+public final class CancelPlanResponse {
+
+    @JsonProperty("plan_id")
+    private final String planId;
+
+    @JsonProperty("status")
+    private final String status;
+
+    @JsonProperty("message")
+    private final String message;
+
+    public CancelPlanResponse(
+            @JsonProperty("plan_id") String planId,
+            @JsonProperty("status") String status,
+            @JsonProperty("message") String message) {
+        this.planId = planId;
+        this.status = status;
+        this.message = message;
+    }
+
+    /**
+     * Returns the ID of the cancelled plan.
+     *
+     * @return the plan ID
+     */
+    public String getPlanId() {
+        return planId;
+    }
+
+    /**
+     * Returns the status after cancellation.
+     *
+     * @return the status (e.g., "cancelled")
+     */
+    public String getStatus() {
+        return status;
+    }
+
+    /**
+     * Returns a human-readable message about the cancellation.
+     *
+     * @return the cancellation message
+     */
+    public String getMessage() {
+        return message;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        CancelPlanResponse that = (CancelPlanResponse) o;
+        return Objects.equals(planId, that.planId) &&
+               Objects.equals(status, that.status) &&
+               Objects.equals(message, that.message);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(planId, status, message);
+    }
+
+    @Override
+    public String toString() {
+        return "CancelPlanResponse{" +
+               "planId='" + planId + '\'' +
+               ", status='" + status + '\'' +
+               ", message='" + message + '\'' +
+               '}';
+    }
+}

src/main/java/com/getaxonflow/sdk/types/CancelPlanResponse.java

@@ -0,0 +1,99 @@
+/*
+ * Copyright 2025 AxonFlow
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.getaxonflow.sdk.types;
+
+import com.fasterxml.jackson.annotation.JsonValue;
+
+/**
+ * Execution mode for multi-agent plan execution.
+ *
+ * <p>Controls how plan steps are scheduled and executed:
+ * <ul>
+ *   <li>{@link #AUTO} - Let the engine determine optimal execution order</li>
+ *   <li>{@link #SEQUENTIAL} - Execute steps strictly in order</li>
+ *   <li>{@link #PARALLEL} - Execute independent steps concurrently</li>
+ *   <li>{@link #BALANCED} - Balance between parallelism and resource usage</li>
+ *   <li>{@link #CONFIRM} - Pause before each step for user confirmation</li>
+ *   <li>{@link #STEP} - Execute one step at a time with manual advancement</li>
+ * </ul>
+ */
+public enum ExecutionMode {
+
+    /**
+     * Let the engine determine optimal execution order.
+     */
+    AUTO("auto"),
+
+    /**
+     * Execute steps strictly in order.
+     */
+    SEQUENTIAL("sequential"),
+
+    /**
+     * Execute independent steps concurrently.
+     */
+    PARALLEL("parallel"),
+
+    /**
+     * Balance between parallelism and resource usage.
+     */
+    BALANCED("balanced"),
+
+    /**
+     * Pause before each step for user confirmation.
+     */
+    CONFIRM("confirm"),
+
+    /**
+     * Execute one step at a time with manual advancement.
+     */
+    STEP("step");
+
+    private final String value;
+
+    ExecutionMode(String value) {
+        this.value = value;
+    }
+
+    /**
+     * Returns the string value used in API requests.
+     *
+     * @return the execution mode value as a string
+     */
+    @JsonValue
+    public String getValue() {
+        return value;
+    }
+
+    /**
+     * Parses a string value to an ExecutionMode enum.
+     *
+     * @param value the string value to parse
+     * @return the corresponding ExecutionMode
+     * @throws IllegalArgumentException if the value is not recognized
+     */
+    public static ExecutionMode fromValue(String value) {
+        if (value == null) {
+            throw new IllegalArgumentException("Execution mode cannot be null");
+        }
+        for (ExecutionMode mode : values()) {
+            if (mode.value.equalsIgnoreCase(value)) {
+                return mode;
+            }
+        }
+        throw new IllegalArgumentException("Unknown execution mode: " + value);
+    }
+}