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/package.json

@@ -28,12 +28,12 @@
   "dependencies": {
     "@affine/reader": "workspace:*",
     "@affine/server-native": "workspace:*",
-    "@ai-sdk/anthropic": "^1.2.12",
-    "@ai-sdk/google": "^1.2.18",
-    "@ai-sdk/google-vertex": "^2.2.23",
-    "@ai-sdk/openai": "^1.3.22",
-    "@ai-sdk/openai-compatible": "^0.2.14",
-    "@ai-sdk/perplexity": "^1.1.9",
+    "@ai-sdk/anthropic": "^2.0.1",
+    "@ai-sdk/google": "^2.0.4",
+    "@ai-sdk/google-vertex": "^3.0.5",
+    "@ai-sdk/openai": "^2.0.10",
+    "@ai-sdk/openai-compatible": "^1.0.5",
+    "@ai-sdk/perplexity": "^2.0.1",
     "@apollo/server": "^4.11.3",
     "@aws-sdk/client-s3": "^3.779.0",
     "@aws-sdk/s3-request-presigner": "^3.779.0",
@@ -75,7 +75,7 @@
     "@prisma/instrumentation": "^6.7.0",
     "@react-email/components": "0.0.38",
     "@socket.io/redis-adapter": "^8.3.0",
-    "ai": "^4.3.4",
+    "ai": "^5.0.10",
     "bullmq": "^5.40.2",
     "cookie-parser": "^1.4.7",
     "cross-env": "^7.0.3",