chore(core): change audio transcription job to use gemini 2.5 pro (#13202)

#### PR Dependency Tree


* **PR #13202** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Improved the "Transcript audio" text action by updating its default AI
model to "gemini-2.5-pro" for enhanced performance.
* Enhanced audio transcription accuracy by refining audio content
handling with a more specific audio format.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: DarkSky <darksky2048@gmail.com>

packages/backend/server/src/__tests__/utils/copilot.ts

@@ -433,7 +433,7 @@ export async function submitAudioTranscription(
   for (const [idx, buffer] of content.entries()) {
     resp = resp.attach(idx.toString(), buffer, {
       filename: fileName,
-      contentType: 'application/octet-stream',
+      contentType: 'audio/opus',
     });
   }