fix(google): allow explicit project for Speech-to-Text

[uuzzrm]

Summary

• add an optional project argument to google.STT
• prefer the explicit project when building the Speech-to-Text v2 recognizer resource
• keep credential-derived project discovery as the fallback for existing callers

Fixes #6772

Validation

• uv run pytest tests/test_google_credentials.py -q
• uv run --group typing mypy --platform linux -p livekit.plugins.google
• uv run ruff check .
• uv run ruff format --check .

The full pytest --unit run reached 1,530 passing tests but also hit unrelated baseline failures/internal errors in the repository's audio decoder, IPC, and concurrent async test infrastructure under Python 3.14.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.