Fix InputBuilder lazy initialization to prevent validation errors (#1444)

* no-issue: Fix eagerly set inputBuilder from/schema to avoid validation errors in runtime

Signed-off-by: Ricardo Zanini <ricardozanini@gmail.com>

* Apply suggestions from code review

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Fix co-pilot comments

Signed-off-by: Ricardo Zanini <ricardozanini@gmail.com>

* Restore db-samples files to match main

---------

Signed-off-by: Ricardo Zanini <ricardozanini@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: ricardozanini

fluent/spec/src/main/java/io/serverlessworkflow/fluent/spec/InputBuilder.java

@@ -29,26 +29,70 @@ public class InputBuilder {
 
   InputBuilder() {
     this.input = new Input();
-    this.input.setFrom(new InputFrom());
-    this.input.setSchema(new SchemaUnion());
   }
 
   public InputBuilder from(String expr) {
+    if (expr == null) {
+      this.input.setFrom(null);
+      return this;
+    }
+
+    if (this.input.getFrom() == null) {
+      this.input.setFrom(new InputFrom());
+    } else {
+      this.input.getFrom().setObject(null);
+    }
+
     this.input.getFrom().setString(expr);
     return this;
   }
 
   public InputBuilder from(Object object) {
+    if (object == null) {
+      this.input.setFrom(null);
+      return this;
+    }
+
+    if (this.input.getFrom() == null) {
+      this.input.setFrom(new InputFrom());
+    } else {
+      this.input.getFrom().setString(null);
+    }
+
     this.input.getFrom().setObject(object);
     return this;
   }
 
   public InputBuilder schema(Object schema) {
+    if (schema == null) {
+      this.input.setSchema(null);
+      return this;
+    }
+
+    if (this.input.getSchema() == null) {
+      this.input.setSchema(new SchemaUnion());
+    } else {
+      // Clear external schema when setting inline schema
+      this.input.getSchema().setSchemaExternal(null);
+    }
+
     this.input.getSchema().setSchemaInline(new SchemaInline(schema));
     return this;
   }
 
   public InputBuilder schema(String schema) {
+    if (schema == null) {
+      this.input.setSchema(null);
+      return this;
+    }
+
+    if (this.input.getSchema() == null) {
+      this.input.setSchema(new SchemaUnion());
+    } else {
+      // Clear inline schema when setting external schema
+      this.input.getSchema().setSchemaInline(null);
+    }
+
     this.input
         .getSchema()
         .setSchemaExternal(
@@ -61,6 +105,23 @@ public InputBuilder schema(String schema) {
     return this;
   }
 
+  public InputBuilder schemaAsJsonString(String schema) {
+    if (schema == null) {
+      this.input.setSchema(null);
+      return this;
+    }
+
+    if (this.input.getSchema() == null) {
+      this.input.setSchema(new SchemaUnion());
+    } else {
+      // Clear external schema when setting inline schema
+      this.input.getSchema().setSchemaExternal(null);
+    }
+
+    this.input.getSchema().setSchemaInline(new SchemaInline(schema));
+    return this;
+  }
+
   public Input build() {
     return this.input;
   }