Address feedback from PR #1 review #2

The initial review of Pull Request #1 has identified several areas for improvement and potential bug fixes. This issue tracks the resolution of these points:

1. Test Coverage for FastAPI Server

The new FastAPI server surface (app factory + routers) lacks test coverage.

• Action: Add integration tests using httpx to exercise key endpoints like /health, /providers, and /prompt.

2. Type Hinting in parse_json_output

The parse_json_output function in src/c3_invoke/output.py accepts None at runtime but is annotated to only accept str.

• Action: Update the signature to def parse_json_output(output: str | None) -> Any:.

3. Exception Guarding in BaseProvider.run()

BaseProvider.run() calls _handle_fallback() without guarding against exceptions that the fallback itself might raise.

• Action: Wrap the _handle_fallback() call in a try/except block to ensure .run() always returns a PromptResponse.

4. Robustness in GeminiProvider._handle_fallback()

The fallback path in GeminiProvider uses subprocess.run(check=True), which can raise CalledProcessError or TimeoutExpired.

• Action: Catch these exceptions within _handle_fallback and return None to allow the caller to handle the failure gracefully.

5. Support Dynamic Output Formats

Both GeminiProvider and ClaudeProvider currently hardcode the output format to text, ignoring the output_format specified in PromptRequest.

• Action: Update build_command() in both providers to honor request.output_format.

Ref: #1