feat(documents): support Gemini native document input

[binsarjr]

What this adds

analyze_document can send supported files to Gemini through its native generateContent endpoint. The document is passed inline with its MIME type, while the user's question and the existing untrusted-document system prompt stay intact.

This also adds the Google Gemini provider preset for both new and existing installations. In this adapter, PDF and RTF are enabled; unsupported binary Office formats are skipped so fallback routing can choose another compatible model.

Review note

Depends on #108. Until that PR is merged, GitHub will also show its parent commits here; the Gemini-specific change is the top commit.

Verification

• Gemini and document-analysis tests: 22 passed
• GitHub Python 3.11 suite: all 3 Gemini tests passed; 2,176 passed and 95 skipped overall, with the same 2 existing dev failures
• Go tests passed
• Python compilation and git diff --check passed