feat(server): adapt gpt5 (#13478)

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

- New Features
- Added GPT-5 family and made GPT-5/-mini the new defaults for Copilot
scenarios and prompts.

- Bug Fixes
- Improved streaming chunk formats and reasoning/text semantics,
consistent attachment mediaType handling, and more reliable reranking
via log-prob handling.

- Refactor
- Unified maxOutputTokens usage; removed per-call step caps and migrated
several tools to a unified inputSchema shape.

- Chores
- Upgraded AI SDK dependencies and bumped an internal dependency
version.

- Tests
- Updated mocks and tests to reference GPT-5 variants and new stream
formats.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/__tests__/mocks/copilot.mock.ts

@@ -57,15 +57,6 @@ export class MockCopilotProvider extends OpenAIProvider {
         },
       ],
     },
-    {
-      id: 'gpt-4.1',
-      capabilities: [
-        {
-          input: [ModelInputType.Text, ModelInputType.Image],
-          output: [ModelOutputType.Text, ModelOutputType.Object],
-        },
-      ],
-    },
     {
       id: 'gpt-4.1-2025-04-14',
       capabilities: [
@@ -76,7 +67,25 @@ export class MockCopilotProvider extends OpenAIProvider {
       ],
     },
     {
-      id: 'gpt-4.1-mini',
+      id: 'gpt-5',
+      capabilities: [
+        {
+          input: [ModelInputType.Text, ModelInputType.Image],
+          output: [ModelOutputType.Text, ModelOutputType.Object],
+        },
+      ],
+    },
+    {
+      id: 'gpt-5-2025-08-07',
+      capabilities: [
+        {
+          input: [ModelInputType.Text, ModelInputType.Image],
+          output: [ModelOutputType.Text, ModelOutputType.Object],
+        },
+      ],
+    },
+    {
+      id: 'gpt-5-mini',
       capabilities: [
         {
           input: [ModelInputType.Text, ModelInputType.Image],