Add Wit.ai transcription provider

[jaxassistant55]

Summary

• add a witai provider for Sapat's current provider-registry architecture
• send binary audio to Wit.ai's /speech HTTP API with configurable API version, endpoint, and content type
• parse both single JSON responses and Wit.ai-style streamed JSON payloads, returning the final transcript text
• document the Wit.ai environment variables and add focused mocked provider/registry coverage

Validation

• .venv/bin/python -m pytest -q -> 185 passed, 1 urllib3 LibreSSL warning
• .venv/bin/black --check sapat/providers/witai.py tests/providers/test_witai.py tests/test_registry.py
• .venv/bin/python -m compileall sapat tests
• git diff --check

No API keys, private media, generated transcripts, or payout details are included.

This is the companion provider PR for my Daytona content attempt on daytona/content#13.