test: explain dead_code allow on common mod in responses_error_headers_test

The attribute is needed because `common::spawn_mock_llm` is never called
from this test binary; narrowing the `use` line does not affect it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011GZ19b5uMpUst42ipnn6tk
Signed-off-by: Sunny Yuan <y.zichen@wustl.edu>

Author: ZichenYuan

crates/agentic-server/tests/responses_error_headers_test.rs

@@ -6,6 +6,8 @@
 //! and processed metadata headers (`retry-after`, request IDs, rate-limit headers) and must not
 //! relabel a non-JSON error body as `application/json`.
 
+// `common` is compiled into every test binary; this one never calls `spawn_mock_llm`,
+// so silence the resulting dead-code warning (same as the other tests that skip it).
 #[allow(dead_code)]
 mod common;