Feature/deepdub plugin - add Deepdub TTS plugin integration

[peternaf]

Summary

• Add a new provider package, livekit-plugins-deepdub, with:
  • plugin registration in livekit.plugins.deepdub
  • Deepdub TTS implementation (TTS + streaming SynthesizeStream)
  • provider metadata/types (models.py, version.py, README.md)
• Wire Deepdub into the monorepo packaging:
  • add workspace package entry
  • add livekit-agents[deepdub] extra
  • include lockfile updates for the new dependency graph
• Add Deepdub test coverage and CI wiring:
  • include Deepdub in TTS provider matrix
  • pass DEEPDUB_API_KEY in CI/tests
  • route Deepdub endpoint in test docker networking

Why non-plugin files changed

• .github/workflows/tests.yml: adds Deepdub provider job + secret env var for integration tests.
• livekit-agents/pyproject.toml and root pyproject.toml: expose/install Deepdub as a workspace extra.
• tests/docker-compose.yml and tests/test_tts.py: add integration test path and provider-specific assertions.
• uv.lock: regenerated to include livekit-plugins-deepdub and transitive deps.

Test plan

• make lint
• make type-check
• uv run pytest tests/test_tts.py -k deepdub
• Local Deepdub synth repro (tests/repro_deepdub_synthesize_local.py) in websocket mode
• Full CI on this PR

Notes

• This PR currently focuses on websocket streaming behavior for Deepdub TTS.

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[peternaf]

The uv.lock file is as it is in origin - I can change it to fix the build error but I'm not sure how to proceed