feat(server): improve context metadata & matching (#12064)

fix AI-20

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

- **New Features**
  - Enhanced file metadata with MIME type, blob ID, and file name across context and workspace, now visible in UI and API.
  - Added workspace-level matching for files and documents with configurable thresholds and workspace scoping in search queries.
  - Introduced a new error type and user-friendly messaging for global workspace context matching failures.

- **Bug Fixes**
  - Improved consistent handling of file MIME types and nullable context IDs for accurate metadata.

- **Documentation**
  - Updated GraphQL schema, queries, and mutations to include new metadata fields, optional parameters, and error types.

- **Style**
  - Added new localization strings for global context matching error messages.

- **Tests**
  - Extended test coverage with new and updated snapshot tests for metadata and matching logic.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/__tests__/models/__snapshots__/copilot-context.spec.ts.md

@@ -6,6 +6,17 @@ Generated by [AVA](https://avajs.dev).
 
 ## should insert embedding by doc id
 
+> should match file embedding
+
+    [
+      {
+        chunk: 0,
+        content: 'content',
+        distance: 0,
+        fileId: 'file-id',
+      },
+    ]
+
 > should return empty array when embedding deleted
 
     []

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

@@ -54,3 +54,18 @@ Generated by [AVA](https://avajs.dev).
         docId: 'new_doc',
       },
     ]
+
+## should insert and search embedding
+
+> should match workspace file embedding
+
+    [
+      {
+        blobId: 'blob1',
+        chunk: 0,
+        content: 'content',
+        distance: 0,
+        mimeType: 'text/plain',
+        name: 'file1',
+      },
+    ]

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

@@ -111,8 +111,7 @@ test('should insert embedding by doc id', async t => {
         1,
         1
       );
-      t.is(ret.length, 1);
-      t.is(ret[0].content, 'content');
+      t.snapshot(ret, 'should match file embedding');
     }
 
     {

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

@@ -107,6 +107,7 @@ test('should insert and search embedding', async t => {
   {
     const { fileId } = await t.context.copilotWorkspace.addFile(workspace.id, {
       fileName: 'file1',
+      blobId: 'blob1',
       mimeType: 'text/plain',
       size: 1,
     });
@@ -129,8 +130,10 @@ test('should insert and search embedding', async t => {
         1,
         1
       );
-      t.is(ret.length, 1);
-      t.is(ret[0].content, 'content');
+      t.snapshot(
+        cleanObject(ret, ['fileId']),
+        'should match workspace file embedding'
+      );
     }
   }
 });