fix(tests): update subprocess config assertion to include agents key

[colombod]

Summary

Fixes a pre-existing test failure on main unrelated to any feature work.

Problem

tests/test_session_spawner_subprocess.py::TestSubprocessRouting::test_subprocess_param_routes_to_subprocess was asserting {"session": {}} but the production code now includes {"agents": {}, "session": {}} — the agents key is always present in the config dict. The test was documented as pre-existing in the PR #186 commit message.

Fix

Updated the assertion at line 94 to match the current actual config shape:

# Before
assert call_kwargs.kwargs["config"] == {"session": {}}
# After
assert call_kwargs.kwargs["config"] == {"agents": {}, "session": {}}

Verification

982/982 tests passing after this fix (zero failures).