refactor(asr): add recognition provider registry

[quanru]

Summary

• add one registry for recognition provider IDs, display names, client factories, credential stores, and auth behavior
• route runtime backend construction and readiness checks through the registry
• populate both recognition-service dropdowns from the same provider order

Validation

• PYTHONPATH=src /Users/bytedance/personal/doubao-say/.venv/bin/python -m unittest tests.unit.test_recognition_providers
• /Users/bytedance/personal/doubao-say/.venv/bin/python -m ruff check src tests packaging
• PYTHONPATH=src /Users/bytedance/personal/doubao-say/.venv/bin/python -m compileall -q src tests/unit/test_recognition_providers.py
• git diff --check

Validation gap

The local macOS environment does not provide the Linux PyGObject gi module, so GTK-dependent tests and the full make check are delegated to Linux CI. This refactor intentionally exposes no new provider or user-visible flow.

Dependency and review order

Root of the recognition-provider stack. Review and merge this PR before the Deepgram transport PR.

Current CI dependency (2026-09-25)

The existing visual failures are from the shared Ark model endpoint returning HTTP 429 RateLimitExceeded.EndpointRPMExceeded; the existing Pages build failure says the published history manifest has an unsupported shape. Both are outside this provider-registry diff. #43 contains the serialized visual runs, retry policy, and Midscene 1.13 report/manifest support and is still under visual validation. After #43 is accepted into main, refresh this stacked branch and rerun its checks before merging #45. The Python core, coverage, package, and contract checks on this PR passed in its last run.