feat(core): add web search tool and reasoning params (#11912)

Close [AI-60](https://linear.app/affine-design/issue/AI-60).

### What changed?
- Add Exa web search tool
- Add reasoning params

packages/backend/server/src/plugins/copilot/config.ts

@@ -16,6 +16,9 @@ declare global {
       unsplash: ConfigItem<{
         key: string;
       }>;
+      exa: ConfigItem<{
+        key: string;
+      }>;
       storage: ConfigItem<StorageProviderConfig>;
       providers: {
         openai: ConfigItem<OpenAIConfig>;
@@ -70,6 +73,12 @@ defineModuleConfig('copilot', {
       key: '',
     },
   },
+  exa: {
+    desc: 'The config for the exa web search key.',
+    default: {
+      key: '',
+    },
+  },
   storage: {
     desc: 'The config for the storage provider.',
     default: {