fix(tests): add conftest.py + fix test_call_peer_errors.py broken imports
- tests/conftest.py: added FakeResponse, client fixture, tmp_token_dir fixture,
and _CaptureHandler (stubs for integration tests)
- tests/test_call_peer_errors.py: rewrote all tests to use existing client.py
patterns (MagicMock session) instead of non-existent httpx fixtures.
Removed mocker/http_mock/conftest fixtures that don't exist in this repo.
12 tests now cover: timeout, connection error, 4xx/5xx errors, empty body,
JSON-RPC envelope format, auth headers, direct→proxy fallback path.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>