Add CAMB.AI transcription provider

[jackjin1997]

Summary

• add a CAMB.AI transcription provider for Sapat's provider registry
• implement CAMB.AI's async flow: create transcription task, poll task status, fetch transcript result
• add CAMB_API_KEY env configuration and mocked provider tests

Validation

• python3 -m compileall sapat tests
• .venv/bin/python -m pytest tests/providers/test_group_b.py -k 'CambAI or Speechmatics or Gladia' -q
• .venv/bin/python -m pytest tests/test_registry.py tests/test_cli.py -q
• .venv/bin/python -m pytest -q
• git diff --check

Daytona bounty context: daytona/content#13

[jackjin1997]

Quick follow-up: this provider PR is still clean/mergeable and has not received review. The companion Daytona guide is daytona/content#295.

Could a maintainer confirm whether new transcription providers are currently accepted? I’m happy to make focused updates if requested; otherwise it is also fine to close it.