fix(runtime): close header propagation gap for LangGraphAgent

The if (agent.headers) guard in configureAgentForRequest silently
skipped header forwarding when agent.headers was undefined (the
default for LangGraphAgent). This meant x-aimock-context, x-test-id,
and other x-* headers were never forwarded to agent backends.

Also wires install_httpx_hook in the Python SDK middleware so
forwarded headers propagate to outgoing LLM API calls.

Closes the gap documented in PR #4773 spec as out-of-scope.