feat: add TEST_EXPRESSION command with request-reply infrastructure

Adds CompletableFuture-based request-reply mechanism for commands that
need synchronous results. CommandReply record in core, pendingReplies
map in AgentRegistryService, test-expression endpoint on config controller
with 5s timeout. CommandAckRequest extended with optional data field.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/dto/CommandAckRequest.java

@@ -6,5 +6,6 @@ package com.cameleer3.server.app.dto;
  *
  * @param status  "SUCCESS" or "FAILURE"
  * @param message human-readable description of the result
+ * @param data    optional structured JSON data returned by the agent (e.g. expression evaluation results)
  */
-public record CommandAckRequest(String status, String message) {}
+public record CommandAckRequest(String status, String message, String data) {}

cameleer3-server-app/src/main/java/com/cameleer3/server/app/dto/TestExpressionRequest.java

@@ -0,0 +1,11 @@
+package com.cameleer3.server.app.dto;
+
+/**
+ * Request body for testing a tap expression against sample data via a live agent.
+ *
+ * @param expression the expression to evaluate (e.g. Simple, JSONPath, XPath)
+ * @param language   the expression language identifier
+ * @param body       sample message body to evaluate the expression against
+ * @param target     what the expression targets (e.g. "body", "header", "property")
+ */
+public record TestExpressionRequest(String expression, String language, String body, String target) {}

cameleer3-server-app/src/main/java/com/cameleer3/server/app/dto/TestExpressionResponse.java

@@ -0,0 +1,9 @@
+package com.cameleer3.server.app.dto;
+
+/**
+ * Response from testing a tap expression against sample data.
+ *
+ * @param result the evaluation result (null if an error occurred)
+ * @param error  error message if evaluation failed (null on success)
+ */
+public record TestExpressionResponse(String result, String error) {}