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/plugins/copilot/config.ts

@@ -57,7 +57,7 @@ defineModuleConfig('copilot', {
         rerank: 'gpt-4.1',
         coding: 'claude-sonnet-4@20250514',
         complex_text_generation: 'gpt-4o-2024-08-06',
-        quick_decision_making: 'gpt-4.1-mini',
+        quick_decision_making: 'gpt-5-mini',
         quick_text_generation: 'gemini-2.5-flash',
         polish_and_summarize: 'gemini-2.5-flash',
       },