Code review found that _sanitize_persisted_blocks only stripped
image_url blocks, causing base64-encoded audio/video payloads to
bloat session history files.
- Extend _sanitize_persisted_blocks to replace input_audio and
video_url blocks with text placeholders using LLMProvider._media_placeholder.
- Add tests for audio/video stripping with and without _meta.path.
All 3267 tests pass.