feat(server): increase embedding jobs concurrency & handle empty content after trim (#12574)

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

- **Improvements**
  - Increased the default concurrency for background tasks, enhancing processing efficiency.
  - Improved handling of empty or unsupported documents to ensure consistent processing.
  - Optimized document filtering to exclude certain documents from processing, improving performance.

- **Bug Fixes**
  - Enhanced detection of empty document summaries, reducing errors during processing.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/__tests__/models/copilot-workspace.spec.ts

@@ -199,6 +199,21 @@ test('should insert and search embedding', async t => {
     );
     t.snapshot(afterAddIgnoreDocs.length, 'should not find docs to embed');
   }
+
+  {
+    const docId = `foo$bar`;
+    await t.context.doc.upsert({
+      spaceId: workspace.id,
+      docId: docId,
+      blob: Uint8Array.from([1, 2, 3]),
+      timestamp: Date.now(),
+      editorId: user.id,
+    });
+    const results = await t.context.copilotWorkspace.findDocsToEmbed(
+      workspace.id
+    );
+    t.false(results.includes(docId), 'docs containing `$` should be excluded');
+  }
 });
 
 test('should check need to be embedded', async t => {