feat(core): use claude-3-7-sonnet as basic chat model (#12036)

Close [AI-59](https://linear.app/affine-design/issue/AI-59)

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

## Summary by CodeRabbit

- **New Features**
  - Added support for two new providers, Anthropic and Exa, in test environments and configurations.
- **Chores**
  - Updated workflow and test environment setup to include API keys for Anthropic and Exa providers.
- **Refactor**
  - Changed the AI model used for the "Chat With AFFiNE AI" prompt to "claude-3-7-sonnet-20250219".

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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

@@ -92,6 +92,12 @@ test.before(async t => {
             perplexity: {
               apiKey: process.env.COPILOT_PERPLEXITY_API_KEY ?? '1',
             },
+            anthropic: {
+              apiKey: process.env.COPILOT_ANTHROPIC_API_KEY ?? '1',
+            },
+          },
+          exa: {
+            key: process.env.COPILOT_EXA_API_KEY ?? '1',
           },
         },
       }),

packages/backend/server/src/plugins/copilot/prompt/prompts.ts

@@ -1037,7 +1037,7 @@ Finally, please only send us the content of your continuation in Markdown Format
 const chat: Prompt[] = [
   {
     name: 'Chat With AFFiNE AI',
-    model: 'gpt-4.1',
+    model: 'claude-3-7-sonnet-20250219',
     messages: [
       {
         role: 'system',