Add mocks for HTTP transport to avoid real network calls during tests.
This addresses reviewer feedback about test reliability:
- Tests are now faster (no network latency)
- Tests are deterministic across environments
- Test intent is clearer (unit testing error handling logic)
The mock throws immediately with a controlled error message,
allowing tests to validate error handling without network dependencies.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
