feat(assemblyai): expose session ID from Begin event

[dlange-aai]

Summary

• Parses the Begin event from the AssemblyAI WebSocket connection, which was previously silently dropped
• Exposes session_id and expires_at as properties on SpeechStream
• Logs the session ID at INFO level on connect for easy debugging

Motivation

The AssemblyAI streaming API sends a Begin event when a session starts containing a unique session ID. Exposing this makes it easy for users to share the session ID with the AssemblyAI team when reporting transcription issues.

Usage

stream = stt.stream()

async for event in stream:
    # session_id is set before any speech events arrive
    print(stream.session_id)   # e.g. "676d673c-83fc-4d8a-bd95-bfe23b1c5a50"
    print(stream.expires_at)   # e.g. 1773775624
    ...

The session ID is also automatically logged:

AssemblyAI session started id=676d673c-83fc-4d8a-bd95-bfe23b1c5a50 expires_at=1773775624

Test plan

• Verify session_id and expires_at are None before connection
• Verify both properties are populated after the first event from a live stream
• Verify session ID appears in logs at INFO level on connect

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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