Honor globalState cache in getAssignable/MentionableUsers when a fetch is already in flight

[Copilot]

getAssignableUsers() and getMentionableUsers() in folderRepositoryManager.ts returned globalStateCache ?? fetchPromise on the first call, but on subsequent calls (when _fetchPromise already existed from a prior invocation) they returned the pending promise directly, bypassing the disk cache and forcing the UI to block on the network — most noticeably after getAssignableUsers(true) clears the in-memory cache while a fetch is still in flight.

Changes

• src/github/folderRepositoryManager.ts: align the fallback return on the "fetch already in progress" path with the first-call path so both methods consistently prefer the globalState cache:

-    return this._fetchMentionableUsersPromise;
+    return globalStateMentionableUsers ?? this._fetchMentionableUsersPromise;

-    return this._fetchAssignableUsersPromise;
+    return globalStateAssignableUsers ?? this._fetchAssignableUsersPromise;

globalStateMentionableUsers / globalStateAssignableUsers are already loaded immediately above each return, so this is a pure correctness fix with no extra I/O. Background refresh behavior is unchanged — concurrent callers now get the cached value instantly while the existing fetch promise continues to update the in-memory and globalState caches when it resolves.