fix(deepgram): recover streaming sessions from unexpected websocket closes

[IM594]

Summary

This PR improves Deepgram streaming recovery when the websocket closes unexpectedly after the handshake has already succeeded.

Changes

• add coverage for unexpected Deepgram websocket closes after handshake
• surface post-handshake abnormal close codes as ASR errors instead of silently treating them as normal completion
• track streaming ASR errors in RecognitionSession
• trigger batch fallback when streaming fails because of websocket close or other streaming errors
• clean up redundant awaits and an unused weak capture in the session pipeline

Why

Previously, if Deepgram closed the websocket unexpectedly after the session was already open, the client could treat that as a normal completion path. In that case, RecognitionSession might skip batch fallback and leave the user with partial or missing text.

Validation

• swift test --filter 'Deepgram|RecognitionSessionTests'
• real Deepgram smoke test for connect/end flow
• real Deepgram smoke test for transcript output using generated speech audio

[joewongjc]

Merged manually into main with conflict resolution. Thanks @IM594! 🎉