Migrate Realtime API to GA protocol with Beta compatibility shim

[jpr5]

Summary

• Migrate aimock's Realtime API handler from deprecated Beta protocol to GA (event renames, nested session config, content type renames)
• Add Beta compatibility shim via OpenAI-Beta: realtime=v1 header detection — thin translation layer in sendEvent() wrapper
• Support 5 new GA models: gpt-realtime-2, gpt-realtime-1.5, gpt-realtime-mini, gpt-realtime-translate, gpt-realtime-whisper
• Add translate/whisper session types with audio buffer handling and model+type validation
• Add image input support (input_image content parts mapped to ChatMessage image_url format)
• Add async function calling commentary phase (phase field on output_item events)
• Add conversation.item.done event, response.cancel handling, endpoint type routing
• Extend drift detection with GA model canary (5 models) and protocol probe (GA vs Beta normalization)
• Update docs, DRIFT.md, competitive matrix

Test plan

• 2965 tests pass (73 realtime integration tests, 66 conformance tests, drift tests gated behind API key)
• TypeScript clean (tsc --noEmit)
• Lint + format clean
• CR converged in 2 rounds (7 agents × 2 = 14 review agents, 4 findings fixed)
• Drift tests against live OpenAI API (requires API key)

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commit: 38157e6