fix(realtime-session): preserve audio format and config fields on agent update

Fixes #317

- Caches and merges last session config to ensure fields like inputAudioFormat/outputAudioFormat are preserved across agent updates
- Adds regression test for Twilio scenario
- Adds changeset for release tracking

Author: myaroshefsky

.changeset/preserve-audio-formats-on-agent-update.md

@@ -0,0 +1,14 @@
+---
+'@openai/agents-realtime': patch
+---
+
+fix(realtime-session): preserve audio format & other session config fields on agent update
+
+Previously, calling `updateAgent()` rebuilt the session config from a minimal subset of fields and
+omitted properties like `inputAudioFormat` / `outputAudioFormat`, `modalities`, `speed`, etc. This
+caused the server to fall back to defaults (e.g. `pcm16`), producing loud static in Twilio calls
+when a custom format (e.g. `g711_ulaw`) was required.
+
+This change caches the last full session config and merges it when generating a new config so
+updates only override dynamic fields (instructions, voice, tools, tracing) while preserving the
+rest. A regression test was added to ensure audio formats persist across `updateAgent` calls.