[pluggable bbr] add integration tests for response plugin execution

[abdallahsamabd]

Follow-up to #2354.
The response plugin execution logic was added in PR #2369, including unit tests for HandleResponseBody and processResponseBody.
This issue tracks adding integration tests that validate response plugin execution over a real gRPC ext_proc stream, covering:
Unary mode: response plugin mutates the response body (e.g., guardrail adding a field)
Unary mode: no response plugins configured results in passthrough behavior
These tests exercise the full Process loop (request headers/body → response headers → response body) to ensure end-to-end correctness.

[abdallahsamabd]

/assign