## PR Title Format

**fix: Fix o3-pro empty response issue by using output_text field**

## Description

This PR fixes a critical bug where o3-pro API calls were returning empty responses. The root cause was incorrect response parsing - the code was trying to manually parse `response.output.content[]` array structure, but o3-pro provides a simpler `output_text` convenience field directly on the response object. This PR also introduces a secure HTTP recording system for testing expensive o3-pro calls.

## Changes Made

- [x] Fixed o3-pro response parsing by using the `output_text` convenience field instead of manual parsing
- [x] Added `_safe_extract_output_text` method with proper validation to handle o3-pro's response format
- [x] Implemented custom HTTP transport recorder to replace respx for more reliable test recordings
- [x] Added comprehensive PII sanitization to prevent accidental API key exposure in test cassettes
- [x] Sanitized all existing test cassettes to remove any exposed secrets
- [x] Updated documentation for the new testing infrastructure
- [x] Added test suite to validate the fix and ensure PII sanitization works correctly

**No breaking changes** - The fix only affects o3-pro model parsing internally.

**Dependencies added:**
- None (uses existing httpx and standard library modules)

## Testing

### Run all linting and tests (required):
```bash
# Activate virtual environment first
source venv/bin/activate

# Run comprehensive code quality checks (recommended)
./code_quality_checks.sh

# If you made tool changes, also run simulator tests
python communication_simulator_test.py
```

- [x] All linting passes (ruff, black, isort)
- [x] All unit tests pass
- [x] **For bug fixes**: Tests added to prevent regression
  - `test_o3_pro_output_text_fix.py` - Validates o3-pro response parsing works correctly
  - `test_o3_pro_http_recording.py` - Tests HTTP recording functionality
  - `test_pii_sanitizer.py` - Ensures PII sanitization works properly
- [x] Manual testing completed with realistic scenarios
  - Verified o3-pro calls return actual content instead of empty responses
  - Validated that recorded cassettes contain no exposed API keys

## Related Issues

Fixes o3-pro API calls returning empty responses on master branch.

## Checklist

- [x] PR title follows the format guidelines above
- [x] **Activated venv and ran code quality checks: `source venv/bin/activate && ./code_quality_checks.sh`**
- [x] Self-review completed
- [x] **Tests added for ALL changes** (see Testing section above)
- [x] Documentation updated as needed
  - Updated `docs/testing.md` with new testing approach
  - Added `docs/vcr-testing.md` for HTTP recording documentation
- [x] All unit tests passing
- [x] Ready for review

## Additional Notes

### The Bug:
On master branch, o3-pro API calls were returning empty responses because the code was trying to parse the response incorrectly:
```python
# Master branch - incorrect parsing
if hasattr(response.output, "content") and response.output.content:
    for content_item in response.output.content:
        if hasattr(content_item, "type") and content_item.type == "output_text":
            content = content_item.text
            break
```

The o3-pro response object actually provides an `output_text` convenience field directly:
```python
# Fixed version - correct parsing
content = response.output_text
```

### The Fix:
1. Added `_safe_extract_output_text` method that properly validates and extracts the `output_text` field
2. Updated the response parsing logic in `_generate_with_responses_endpoint` to use this new method
3. Added proper error handling and validation to catch future response format issues

### Additional Improvements:
- **Testing Infrastructure**: Implemented HTTP transport recorder to enable testing without repeated expensive API calls
- **Security**: Added automatic PII sanitization to prevent API keys from being accidentally committed in test recordings

### Development Notes:
- During development, we encountered timeout issues with the initial respx-based approach which led to implementing the custom HTTP transport recorder
- The transport recorder solution properly handles streaming responses and gzip compression

### For Reviewers:
- The core fix is in `providers/openai_compatible.py` lines 307-335 and line 396
- The HTTP transport recorder is test infrastructure only and doesn't affect production code
- All test cassettes have been sanitized and verified to contain no secrets