[Bug Fix] Surface Thread-kind items in user submission history (#284)
* [fix] Surface Thread-kind items in user submission history
Thread items submitted with a schema field role were
silently being dropped from creator-keyed surfaces
* [test] Add unit tests for getCreator across item kinds
Pins the behavior change from the previous commit so the THREAD case
can't silently regress to `undefined` again. Covers:
- CONTENT: returns the creator from the creatorId field role; returns
undefined when the role is unconfigured or the field is missing.
- THREAD: same as CONTENT (this is the regression case the bug fix
targets).
- USER: always returns undefined.
Exports `getCreator` from `makeItemSubmission.ts` for testability;
nothing else in the codebase imports it.