Bug 1: _drain_pending did not call extract_documents on follow-up
messages arriving mid-turn. Documents attached to queued messages were
silently dropped because _build_user_content only handles images.
Fix: call extract_documents before _build_user_content in _drain_pending.
Bug 2: extract_documents read the entire file into memory (up to 50 MB)
just to check 16 bytes of magic header for MIME detection.
Fix: read only the first 16 bytes via open()+read(16) instead of
Path.read_bytes().
Added regression tests for both bugs.
Made-with: Cursor