feat(tts): integrate Gemini API text-to-speech

[RayJiang4S]

Summary

• Replace the Google TTS provider with Gemini API text-to-speech (gemini-3.1-flash-tts-preview)
• Support API-key auth via GEMINI_API_KEY / GOOGLE_API_KEY
• Add prompt-directed single-speaker and two-speaker payload support, writing Gemini PCM output as WAV
• Update provider docs, environment examples, and contract coverage for the Gemini API workflow

Why

Google's Gemini API TTS docs show gemini-3.1-flash-tts-preview as a supported TTS model with responseModalities: [\"AUDIO\"], prebuilt voice config, 24 kHz PCM output, and single/multi-speaker support. This keeps OpenMontage aligned with the current Google TTS path while preserving the existing provider contract and registry discovery style.

Verification

• Real smoke test passed with gemini-3.1-flash-tts-preview / Kore, producing a 24 kHz mono WAV (5.24s)
• .venv/bin/python -m pytest tests/contracts/test_phase3_contracts.py -q -> 70 passed
• .venv/bin/python -m compileall tools/audio/google_tts.py tests/contracts/test_phase3_contracts.py -> passed
• git diff --check -> passed

[RayJiang4S]

Updated this PR with the Google TTS workflow learnings from a real narration audition pass.

What changed:

• Added delivery_preset support to google_tts for reusable Gemini prompt-directed narration styles:
  • technical_briefing
  • compact_explainer
  • warm_opening
  • clear_cta
• Added duration_target_seconds as explicit prompt guidance for approximate timing. This does not pretend Gemini exposes a hard speed parameter; it records timing intent in the synthesis prompt and output metadata.
• Returned delivery_preset and duration_target_seconds in provider output metadata for auditability.
• Documented the recommended preset + duration workflow in docs/PROVIDERS.md.
• Added contract coverage for prompt generation and invalid preset/duration validation.

Validation:

• .venv/bin/python -m pytest tests/contracts/test_phase3_contracts.py -q passed: 73 tests.
• .venv/bin/python -m py_compile tools/audio/google_tts.py tests/contracts/test_phase3_contracts.py passed.
• git diff --check passed.