feat(uploads): add pymupdf4llm PDF converter with auto-fallback and async offload (#1727)

* feat(uploads): add pymupdf4llm PDF converter with auto-fallback and async offload

- Introduce pymupdf4llm as an optional PDF converter with better heading
  detection and table preservation than MarkItDown
- Auto mode: prefer pymupdf4llm when installed; fall back to MarkItDown
  when output is suspiciously sparse (image-based / scanned PDFs)
- Sparsity check uses chars-per-page (< 50 chars/page) rather than an
  absolute threshold, correctly handling both short and long documents
- Large files (> 1 MB) are offloaded to asyncio.to_thread() to avoid
  blocking the event loop (related: #1569)
- Add UploadsConfig with pdf_converter field (auto/pymupdf4llm/markitdown)
- Add pymupdf4llm as optional dependency: pip install deerflow-harness[pymupdf]
- Add 14 unit tests covering sparsity heuristic, routing logic, and async path

* fix(uploads): address Copilot review comments on PDF converter

- Fix docstring: MIN_CHARS_PYMUPDF -> _MIN_CHARS_PER_PAGE (typo)
- Fix file handle leak: wrap pymupdf.open in try/finally to ensure doc.close()
- Fix silent fallback gap: _convert_pdf_with_pymupdf4llm now catches all
  conversion exceptions (not just ImportError), so encrypted/corrupt PDFs
  fall back to MarkItDown instead of propagating
- Tighten type: pdf_converter field changed from str to Literal[auto|pymupdf4llm|markitdown]
- Normalize config value: _get_pdf_converter() strips and lowercases the raw
  config string, warns and falls back to 'auto' on unknown values

backend/packages/harness/pyproject.toml

@@ -34,6 +34,9 @@ dependencies = [
     "langgraph-sdk>=0.1.51",
 ]
 
+[project.optional-dependencies]
+pymupdf = ["pymupdf4llm>=0.0.17"]
+
 [build-system]
 requires = ["hatchling"]
 build-backend = "hatchling.build"