fix(a2a): align A2A endpoint with JSON-RPC 2.0 wire format changes

- Fix asgi_app.py: use a2a_request.method instead of a2a_request.operation
  (A2aRequest model was updated in master to use JSON-RPC 2.0 field names)
- Fix server_lifecycle_steps.py: send JSON-RPC 2.0 wire format with
  "method" field; check result.status instead of top-level status
- Fix server_lifecycle.feature: update step name to match new step
  definition (A2A response result status)
- Fix helper_server_lifecycle.py: use "method" field in JSON-RPC 2.0
  payload; check result.status in response

robot/helper_server_lifecycle.py

@@ -60,11 +60,18 @@ def _test_a2a_dispatch() -> None:
 
     app = create_asgi_app()
     client = TestClient(app)
-    payload = {"jsonrpc": "2.0", "method": "_cleveragents/health/check", "params": {}}
+    # JSON-RPC 2.0 wire format: use "method" field
+    payload = {
+        "jsonrpc": "2.0",
+        "method": "_cleveragents/health/check",
+        "params": {},
+    }
     resp = client.post("/a2a", json=payload)
     assert resp.status_code == 200
     data = resp.json()
-    assert data["result"]["status"] == "healthy"
+    # JSON-RPC 2.0 response: result is nested under "result"
+    result = data.get("result") or {}
+    assert result.get("status") == "healthy", f"Unexpected response: {data}"
     print("a2a-dispatch-ok")