fix: report thread endorsement in list responses on the MySQL backend

[rabeeh-ta]

On the MySQL backend, a question with an endorsed answer shows no answered indicator anywhere a thread list is rendered. Opening the post shows it correctly. The two disagree because the two code paths compute the flag differently.

1. MySQLBackend.get_endorsed() returns a dict keyed by strings: {str(thread_id): True for ...}.
2. MySQLBackend.threads_presentor() reads it back with an integer: threads_endorsed.get(thread.pk, False). thread.pk is Django's implicit auto id, so the lookup never matches and every thread in a list response is presented as endorsed: False.
3. The two adjacent lookups in the same loop already convert (read_states.get(str(thread.pk), ...) and threads_flagged.get(str(thread.pk), 0)); only the endorsement one does not.
4. Fetching a single thread is unaffected: to_dict() emits no endorsed key, so ThreadSerializer.get_endorsed() computes it fresh from obj["_id"], which is already a string, and returns the right answer. That is why the flag appears once a post is opened.

The MongoDB backend is not affected: its threads_presentor() uses one string thread_key throughout.

Downstream, has_endorsed in the edx-platform thread serializer is source="endorsed", and frontend-app-discussions renders its answered indicator on hasEndorsed && type === 'question'. So every MySQL-backend site loses that indicator in the posts list.

The line is unchanged since the first release and is present in v0.4.7. There was no test covering has_endorsed in a list response, which is why it went unnoticed; this adds one that fails before the fix and passes after, mirroring the existing test_computes_endorsed_correctly for the single-thread path.

Manual testing instructions:

1. On a MySQL-backend course, create a question post and post a response to it.
2. Mark the response as the answer.
3. Reload the discussions posts list without opening the post. Before this change the post shows no answered indicator; after it, it does.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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[rabeeh-ta]

@openedx-webhooks, I have signed the CLA.