feat: get prompt model names (#13607)

fix AI-419

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

- New Features
- New API to fetch available models for a prompt, returning default,
optional, and pro models with human‑readable names.
- Added temperature and topP settings to prompt configuration for finer
control.
- Refactor
- When no model is chosen, the default model is used instead of
auto-picking a pro model.
- Model metadata across providers now includes readable names, improving
listings and selection UX.
- Tests
- Updated test snapshots and descriptions to reflect the new
default-model behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/__tests__/__snapshots__/copilot.spec.ts.md

@@ -467,9 +467,9 @@ Generated by [AVA](https://avajs.dev).
 
     'gemini-2.5-flash'
 
-> should pick first pro model when no requested model during active
+> should pick default model when no requested model during active
 
-    'gemini-2.5-pro'
+    'gemini-2.5-flash'
 
 > should honor requested pro model during active
 

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

@@ -2129,13 +2129,13 @@ test('should resolve model correctly based on subscription status and prompt con
     );
   }
 
-  // payment enabled + active: without requested -> first pro; requested pro should be honored
+  // payment enabled + active: without requested -> default model; requested pro should be honored
   {
     mockStatus(SubscriptionStatus.Active);
     const model6 = await s.resolveModel(true);
     t.snapshot(
       model6,
-      'should pick first pro model when no requested model during active'
+      'should pick default model when no requested model during active'
     );
 
     const model7 = await s.resolveModel(true, 'claude-sonnet-4@20250514');