test(ReadFileTool): add unit tests for image handling, error paths, approval flow

Adds comprehensive tests for ReadFileTool covering:
- Input validation (missing path parameter)
- RooIgnore blocking
- Directory read error handling
- Image handling (memory limits, format detection, model support)
- Binary file handling (PDF, DOCX, unsupported formats)
- Text file processing (slice and indentation modes)
- Approval flow (approve, deny, feedback)
- Output structure formatting
- Error handling (file read errors, stat errors)

29 new tests covering the orchestration layer that was previously
untested after the Codex-inspired refactor.

Author: hannesrudolph