- Add field-typing helpers (parse_bool, parse_float, parse_int, parse_comma_list)
- Add validate_pdf_magic_bytes() to check for %PDF- header
- Update ExtractParams to support: ocr_language, ocr_dpi, markdown_anchors
- Update receive_pdf() to use type-aware parsing and validate PDF bytes
- Update build_options() to map form fields to ExtractionOptions
- Add comprehensive unit tests for form helpers and build_options
Per plan section 2127-2137, implements optional form field parsing with:
- Forward-compatibility for unknown fields (warning logs, ignored)
- Clear 400 errors with hints on parse failure
- Typed coercion (bool from "true"/"1"; comma-list to Vec<String>)
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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