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/common/graphql/src/schema.ts

@@ -569,6 +569,7 @@ export interface CopilotWorkspaceIgnoredDocTypeEdge {
 
 export interface CreateChatMessageInput {
   attachments?: InputMaybe<Array<Scalars['String']['input']>>;
+  blob?: InputMaybe<Scalars['Upload']['input']>;
   blobs?: InputMaybe<Array<Scalars['Upload']['input']>>;
   content?: InputMaybe<Scalars['String']['input']>;
   params?: InputMaybe<Scalars['JSON']['input']>;