[release/v1.2] fix: validate projection engineVersion and V2 trackEmittedStreams combo (#391)

* fix: validate projection engineVersion and V2 trackEmittedStreams combo

Reject engineVersion values outside the documented set (0/1/2) and the
V2 + trackEmittedStreams combination before sending the request.

* fix: deliver projection validation error via exceptional completion

Match how server-side failures of the same condition surface; keep the
public CompletableFuture API free of synchronous throws.

* style: rename failed future variable to result for consistency

---------

Co-authored-by: William Chong <william-chong@outlook.com>

Author: github-actions[bot]

src/main/java/io/kurrent/dbclient/CreateProjection.java

@@ -29,6 +29,13 @@ public CreateProjection(final GrpcClient client, final String projectionName, fi
 
     @SuppressWarnings("unchecked")
     public CompletableFuture execute() {
+        if (engineVersion == 2 && trackEmittedStreams) {
+            CompletableFuture<Projectionmanagement.CreateResp> result = new CompletableFuture<>();
+            result.completeExceptionally(new IllegalArgumentException(
+                    "trackEmittedStreams is not supported when engineVersion is 2 (V2)"));
+            return result;
+        }
+
         return this.client.run(channel -> {
             Projectionmanagement.CreateReq.Options.Continuous.Builder continuousBuilder =
                     Projectionmanagement.CreateReq.Options.Continuous.newBuilder()

src/main/java/io/kurrent/dbclient/CreateProjectionOptions.java

@@ -54,8 +54,14 @@ public CreateProjectionOptions emitEnabled(boolean value) {
      * The engine version is pinned at create time and cannot be changed via update.
      * V2 has limitations versus V1: {@code trackEmittedStreams} is rejected,
      * result streams are not emitted, and bi-state projections are not supported.
+     *
+     * @throws IllegalArgumentException if {@code value} is not {@code 0}, {@code 1}, or {@code 2}.
      */
     public CreateProjectionOptions engineVersion(int value) {
+        if (value < 0 || value > 2) {
+            throw new IllegalArgumentException(
+                    "engineVersion must be 0, 1, or 2 (got " + value + ")");
+        }
         this.engineVersion = value;
         return this;
     }