Add AssemblyAI transcription provider

[chinook1001]

Summary

• add an AssemblyAI transcription provider for the current provider-registry architecture
• support AssemblyAI upload + transcript submission + polling, comma-separated speech model fallback, language detection, prompt passthrough, and optional speaker/punctuation formatting flags
• document AssemblyAI environment variables and cover the provider with mocked async-flow, availability, registry, and failure-path tests

Validation

• PYTHONPATH=/Users/lucas/Documents/codex-bounty-work/sapat /tmp/sapat-test-venv/bin/python -m pytest tests -q -> 183 passed, 1 LibreSSL urllib3 warning
• PYTHONPATH=/Users/lucas/Documents/codex-bounty-work/sapat /tmp/sapat-test-venv/bin/python -m black --check sapat/providers/assemblyai.py sapat/providers/async_poll.py tests/providers/test_group_b.py tests/test_registry.py
• python3 -m compileall sapat/providers/assemblyai.py
• git diff --check

Companion implementation for Daytona content bounty daytona/content#13. No secrets, private audio, generated transcripts, or payout details are included.

[chinook1001]

Update: added a Signed-off-by line to the commit after opening the PR.

Local validation remains green:

• PYTHONPATH=/Users/lucas/Documents/codex-bounty-work/sapat /tmp/sapat-test-venv/bin/python -m pytest tests -q -> 183 passed, 1 local LibreSSL/urllib3 warning
• PYTHONPATH=/Users/lucas/Documents/codex-bounty-work/sapat /tmp/sapat-test-venv/bin/python -m black --check sapat/providers/assemblyai.py sapat/providers/async_poll.py tests/providers/test_group_b.py tests/test_registry.py
• python3 -m compileall sapat/providers/assemblyai.py
• git diff --check

The current Continue status checks appear to be failing without PR feedback, similar to neighboring provider PR #59. I do not see maintainer-requested changes yet.

[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.