Add IBM Watson STT provider

[pqmashiro-commits]

Summary

• add an ibm_watson speech-to-text provider using IBM Watson Speech to Text's REST /v1/recognize endpoint
• document the required IBM_WATSON_STT_API_KEY and IBM_WATSON_STT_URL settings plus --provider ibm_watson usage
• add mocked provider tests for request construction, content type override, model aliasing, availability checks, and API errors

This is the companion Sapat implementation PR for a Daytona content guide submission in daytona/content#13. No API keys, private audio, or generated transcripts are committed.

Validation

• .\.venv\Scripts\python.exe -m pytest tests\test_base.py tests\test_cli.py tests\test_registry.py tests\providers\test_group_a.py tests\providers\test_group_b.py tests\providers\test_group_c.py -q (149 passed)
• .\.venv\Scripts\python.exe -m black --check --target-version py314 sapat\providers\ibm_watson.py tests\providers\test_group_a.py
• .\.venv\Scripts\python.exe -m compileall -q sapat tests
• git diff --check

[pqmashiro-commits]

Closing this to avoid a duplicate submission: an IBM Watson Speech to Text implementation and Daytona companion guide are already open in #62 and daytona/content#266. I will avoid claiming the same provider path.