fix: report endorsed status for threads in MySQL thread lists

[AhtishamShahid]

Description

With the MySQL backend, a question with an endorsed ("answered") response is always listed as unanswered.

MySQLBackend.get_endorsed() returns a dict keyed by str(thread_id):

return {str(thread_id): True for thread_id in endorsed_thread_ids}

threads_presentor() looked that dict up with the integer primary key, so the lookup always missed and every thread came back with endorsed: False:

is_endorsed = threads_endorsed.get(thread.pk, False)  # 5 != "5"

This PR looks the thread up by str(thread.pk), which matches the read_states and threads_flagged lookups next to it. The MongoDB backend is not affected because it uses string ids on both sides.

User-facing impact

In the discussions MFE, marking a response as the answer shows the green "answered" checkmark in the post list. That's because the MFE re-fetches the single thread, which does not go through threads_presentor. After a page reload, the list is rebuilt from the thread-list endpoint, which returned has_endorsed: false, so the checkmark disappeared. Course staff then had no way to see at a glance which questions still needed an answer.

Testing

• Added test_threads_presentor_includes_endorsed_status. It fails on master and passes with this change.
• Full unit suite passes (pytest --ignore=tests/e2e: 203 passed). pylint and mypy are clean. isort is clean on the changed files.

Manual testing instructions

1. In an Open edX (Tutor) dev environment, enable forum_v2.enable_mysql_backend for a course.
2. In the discussions MFE, as course staff, create a question post and add a response.
3. Mark the response as the answer. The green checkmark appears in the post list.
4. Reload the page.
   • Before this change: the checkmark disappears, and GET /api/discussion/v1/threads/?course_id=... returns "has_endorsed": false for the thread.
   • After this change: the checkmark stays, and the list returns "has_endorsed": true.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added (N/A, the repo has no changelog)
• Documentation updated (not only docstrings) (N/A)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets (no migrations or new dependencies)

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[Copilot]

Copilot review overview

🟢 Approval recommended

The focused fix aligns ID types and is adequately covered by a regression test.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes MySQL thread lists to preserve answered/endorsed status after reload.

Changes:

• Uses string thread IDs for endorsed-status lookup.
• Adds regression coverage for answered and unanswered threads.

File	Description
src/​forum/​backends/​mysql/​api.py	Corrects endorsed-status lookup key type.
tests/​test_backends/​test_mysql/​test_api.py	Adds regression test for endorsement presentation.

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[AhtishamShahid]

Hi , @taimoor-ahmed-1 can you please merge this PR? I don't have write access to this repo. Thanks.