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/plugins/copilot/embedding/client.ts

@@ -7,7 +7,11 @@ import {
   CopilotProviderNotSupported,
 } from '../../../base';
 import { CopilotFailedToGenerateEmbedding } from '../../../base/error/errors.gen';
-import { ChunkSimilarity, Embedding } from '../../../models';
+import {
+  ChunkSimilarity,
+  Embedding,
+  EMBEDDING_DIMENSIONS,
+} from '../../../models';
 import { PromptService } from '../prompt';
 import {
   type CopilotProvider,
@@ -16,11 +20,7 @@ import {
   ModelInputType,
   ModelOutputType,
 } from '../providers';
-import {
-  EMBEDDING_DIMENSIONS,
-  EmbeddingClient,
-  type ReRankResult,
-} from './types';
+import { EmbeddingClient, type ReRankResult } from './types';
 
 const EMBEDDING_MODEL = 'gemini-embedding-001';
 const RERANK_PROMPT = 'Rerank results';