feat: add Gemini transcription provider

[xhh678876]

Summary

• Adds a Google Gemini Developer API transcription backend available through --api gemini.
• Sends converted audio as inline base64 to Gemini generateContent with GEMINI_API_KEY, optional GEMINI_MODEL, and no new runtime dependencies.
• Documents Gemini environment variables and usage in the README.
• Adds mocked unittest coverage for request construction, response parsing, validation, CLI routing, and error paths.

Why

This supports daytonaio/content#13 by adding a new non-overlapping Sapat provider for a companion Daytona guide.

Validation

• python -m venv .venv && .venv/bin/python -m pip install -q -e .
• .venv/bin/python -m unittest discover -s tests -v (12 tests)
• .venv/bin/python -m compileall src tests
• git diff --check

AI-assisted with Codex and reviewed with Claude Code; I inspected and validated the final diff locally.

[xhh678876]

Note on external Continuous AI checks

The three Continuous AI status contexts currently report Agent encountered an error and their target URLs return unavailable/404 pages from outside GitHub. I could not retrieve actionable failure logs from those links.

Local validation for this PR passed:

• .venv/bin/python -m unittest discover -s tests -v (12 tests)
• .venv/bin/python -m compileall src tests
• git diff --check

I am happy to address any concrete maintainer or bot feedback if logs become available.

[nibzard]

Thank you for this contribution! Your implementation was reviewed as part of a batch review of all incoming provider PRs.

We've decided to take a different approach: instead of merging individual provider PRs (which all targeted a different code architecture than what exists on main), we've refactored the entire codebase into a proper provider plugin architecture with auto-discovery and shared mixins. Your provider's API integration patterns were used as reference when building the Gemini provider in the new architecture.

The refactor is on branch feat/provider-plugin-architecture and includes 29 providers, 176 tests, and a clean plugin system. We appreciate your contribution — it helped shape the final design. Closing this in favor of the unified approach.