Function UserScopedCacheKey

UserScopedCacheKey(
userID: string | null | undefined,
...parts: string[],
): string | null
Builds a cache key for user-scoped data from the entry's own key parts plus the user segment (UserCacheKeySegment). Returns null when there is no user; callers must then skip the cache for both read and write.
Parameters
- userID: string | null | undefined
  the ID of the user the data was loaded for
- ...parts: string[]
  the parts that identify the entry apart from the user, in a fixed order
Returns string | null
the parts and the user segment joined with :, or null when userID is empty
Example
```
UserScopedCacheKey(contextUser.ID, agentID, sourceName) // '<agentID>:<sourceName>:u:<userID>'
```
- Defined in util/UserScopedCacheKey.ts:48