* Add file upload API with parser selection and conversion hooks
- Add FileRetainRequest.parser field for per-request parser selection
- Add FileConvertResult dataclass and on_file_convert_complete extension hook
- Fire hook after file-to-markdown conversion with output text for metering
- Fix obstore.Bytes incompatibility with httpx in Iris parser (GCS returns
obstore.Bytes instead of plain bytes)
- Export new types from extensions __init__
* remove parser field from FileRetainRequest API
Parser selection remains server-side only via HINDSIGHT_API_FILE_PARSER config.
* test: add tests for on_file_convert_complete extension hook
Verifies that the hook is called with correct parameters on success,
called once per file for multi-file uploads, and not called when
file conversion fails.
* test: verify tenant_id propagation to on_file_convert_complete hook
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Co-authored-by: Nicolò Boschi <boschi1997@gmail.com>
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