fix: Add transcription session created event

[richiejp]

OpenAI first sends a transcription session created event when specifying
the transcription intent, but we only handle transcription session updated.

Needed to fix this: richiejp/VoxInput#22

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.11%. Comparing base (83067fc) to head (bdc12c6).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
server_event.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   62.30%   62.11%   -0.20%     
==========================================
  Files          14       14              
  Lines         650      652       +2     
==========================================
  Hits          405      405              
- Misses        233      235       +2     
  Partials       12       12              

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