Add Fireworks transcription provider

[chinook1001]

Summary

• add a Fireworks AI transcription provider using the direct Audio API REST endpoint
• route whisper-v3 and whisper-v3-turbo to the documented serverless audio endpoints
• document FIREWORKS_API_KEY in .env.example
• add mocked provider tests for request shape, model aliases, availability, endpoint routing, errors, and empty responses

Validation

• PYTHONPATH=/Users/lucas/Documents/codex-bounty-work/sapat-fireworks /tmp/sapat-eleven-venv/bin/python -m pytest tests/providers/test_fireworks.py tests/test_registry.py -q
• PYTHONPATH=/Users/lucas/Documents/codex-bounty-work/sapat-fireworks /tmp/sapat-eleven-venv/bin/python -m black --check sapat/providers/fireworks.py tests/providers/test_fireworks.py
• PYTHONPATH=/Users/lucas/Documents/codex-bounty-work/sapat-fireworks /tmp/sapat-eleven-venv/bin/python -m compileall sapat/providers/fireworks.py tests/providers/test_fireworks.py
• git diff --check
• registry smoke with FIREWORKS_API_KEY=test confirms fireworks is discoverable

No API keys or private audio are included.

[chinook1001]

Validation update for current head 37c9cb8c0f8defb4063040b882f316e894f0beaf:

• PYTHONPATH=/Users/lucas/Documents/codex-bounty-work/sapat-fireworks /tmp/sapat-verify-venv/bin/python -m pytest tests -q -> 183 passed, 1 local LibreSSL/urllib3 warning.
• PYTHONPATH=/Users/lucas/Documents/codex-bounty-work/sapat-fireworks /tmp/sapat-verify-venv/bin/python -m pytest tests/providers/test_fireworks.py tests/test_registry.py -q -> 17 passed, 1 local LibreSSL/urllib3 warning.
• PYTHONPATH=/Users/lucas/Documents/codex-bounty-work/sapat-fireworks /tmp/sapat-verify-venv/bin/python -m black --check sapat/providers/fireworks.py tests/providers/test_fireworks.py -> pass.
• git diff --check origin/main...HEAD -> pass.

The combined GitHub status is currently red because the external Continuous AI agent statuses are still pending/failing from agent errors; I do not see a failing repository test/check-run attached to this head. No secrets or live Fireworks credentials were used; provider behavior is covered with mocked HTTP calls.

[chinook1001]

Note on current failing GitHub status: the failing contexts are from the external Continuous AI agents reporting Agent encountered an error, not from the project test suite. I rechecked the branch locally and posted the validation commands/results above; happy to address any maintainer-visible code/test failure if one appears.