fix: order responses numerically by PK instead of lexicographically

[taimoor-ahmed-1]

Summary

Comment.get_list() sorted responses by sort_key, a CharField that holds the comment's primary key rendered as a string. String comparison gives the lexicographic order \"1\", \"10\", \"11\", \"12\", \"2\", \"3\", … instead of the numeric 1, 2, 3, …, 12. To users this looks as though the response sort dropdown does nothing — picking Newest first or Oldest first produces what appears to be a random shuffle.

This change orders comments at the database layer by integer PK (pk is monotonic with creation time, so PK ascending == oldest first).

Fixes openedx/frontend-app-discussions#823.
Discuss thread (same user reporting): https://discuss.openedx.org/t/discuss-forum-messages-order-not-organized-as-expected-in-teak/18665

What changed

In `forum/backends/mysql/models.py`, `Comment.get_list()`:

• Before: Pulled all comments into Python and called `sorted(..., key=lambda x: (x.sort_key is None, x.sort_key or ""))` — string sort.
• After: Uses Django's `queryset.order_by("pk")` / `order_by("-pk")` so MySQL does the ordering numerically.

Behaviour for the two documented inputs is preserved:

• `sort == 1` (ascending / Oldest first) → `order_by("pk")`
• `sort == -1` (descending / Newest first) → `order_by("-pk")`
• `sort is None` (legacy unsorted call) → now also `order_by("pk")`. This is a behaviour change but fixes a latent bug: when `sort=None` and `resp_limit` was set, the old code sliced into an empty `result` list and returned no rows at all. The new code returns a deterministic page.

Why not fix `get_sort_key` to be int-safe instead?

`sort_key` is a denormalised `CharField` of the form `"{pk}"` or `"{parent_pk}-{pk}"`. Sorting by PK avoids the field entirely, removes a Python-level pass over every result, and pushes ordering to the database (so pagination via `[resp_skip:resp_end]` runs against an already-sorted queryset instead of a list slice).

Endorsed/answered responses

The reporter notes that endorsed/answer responses should remain on top regardless of sort. That separation is handled one layer up in edx-platform `lms/djangoapps/discussion/rest_api/api.py` for question threads (`endorsed_responses` + `non_endorsed_responses`). Not in scope for this fix.

Test plan

• Local Tutor dev (`forum_v2.enable_mysql_backend` on) — created a thread with 12+ responses; sort dropdown now produces correct numeric order in both directions.
• Suggested follow-up unit test: in `tests/test_backends/test_mysql/test_comments.py` create ≥10 comments and assert `Comment.get_list(sort=1, comment_thread=…)` returns ascending PK order (not lex order). Happy to add it in this PR if reviewers prefer it bundled.

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