feat(server): add compatibility for ios client (#13263)

fix AI-355

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

## Summary by CodeRabbit

* **New Features**
* Added support for uploading a single file as an attachment when
creating chat messages, in addition to existing multiple file uploads.

* **Tests**
* Expanded test coverage to verify message creation with both single and
multiple file attachments.

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

packages/backend/server/src/schema.gql

@@ -457,6 +457,7 @@ type CopilotWorkspaceIgnoredDocTypeEdge {
 
 input CreateChatMessageInput {
   attachments: [String!]
+  blob: Upload
   blobs: [Upload!]
   content: String
   params: JSON