fix(server): fulfill empty embedding for trashed docs (#13461)

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

## Summary by CodeRabbit

- New Features
  - None
- Bug Fixes
- Ensures a placeholder embedding is always created when content is
empty or after deletion, reducing errors and improving Copilot
stability.
- Refactor
- Centralized empty-embedding handling for consistent behavior across
workflows.
- Standardized embedding dimension configuration to a single source for
reliability.
- Chores
- Simplified internal embedding module surface and imports for
maintainability.

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

packages/backend/server/src/models/copilot-context.ts

@@ -19,6 +19,8 @@ import {
 
 type UpdateCopilotContextInput = Pick<CopilotContext, 'config'>;
 
+export const EMBEDDING_DIMENSIONS = 1024;
+
 /**
  * Copilot Job Model
  */
@@ -290,10 +292,24 @@ export class CopilotContextModel extends BaseModel {
     `;
   }
 
+  async fulfillEmptyEmbedding(workspaceId: string, docId: string) {
+    const emptyEmbedding = {
+      index: 0,
+      content: '',
+      embedding: Array.from({ length: EMBEDDING_DIMENSIONS }, () => 0),
+    };
+    await this.models.copilotContext.insertWorkspaceEmbedding(
+      workspaceId,
+      docId,
+      [emptyEmbedding]
+    );
+  }
+
   async deleteWorkspaceEmbedding(workspaceId: string, docId: string) {
     await this.db.aiWorkspaceEmbedding.deleteMany({
       where: { workspaceId, docId },
     });
+    await this.fulfillEmptyEmbedding(workspaceId, docId);
   }
 
   async matchWorkspaceEmbedding(