(chore) Add regression and acceptance tests for the mounts file and archive endpoints

[mmabrouk]

The drive/mounts work added new behavior that ships with thin regression coverage. The pre-merge fixes (#5411/#5412) add tests for the specific bugs they close, but two gaps remain worth pinning.

The goal is to lock the highest-value untested behaviors against regression.

• Add an archive/mtime regression test. The mtime bug this PR fixed silently truncated downloads to zero bytes with an HTTP 200 — the single strongest pinning-test candidate in the whole change.
• Add acceptance coverage for the new file endpoints (the shallow/with-counts listing and the download-all archive) in the mounts acceptance suite (api/oss/tests/pytest/acceptance/mounts/); today it gained nothing for them.
• Add unit tests for _rollup_recent_entries if the pre-merge tests did not already cover it — a pure List[MountFile] -> List[MountFile] function with subtle boundary semantics.

Follow-up from the #5400 backend review (test-scaffolding fixes stacked in #5411/#5412). Cited findings: 2.4 (the new endpoints' behavior was untested and the PR body overstated coverage) and 3.3 (_rollup_recent_entries is clever, silent in edge cases, and had zero tests). The review report is not in the repo, so this summary stands alone.

[linear-code]

AGE-3977

[Yuvakunaal]

Hi, I'd like to work on this — planning to add the archive/mtime regression test, the acceptance coverage, and unit tests for _rollup_recent_entries."

[mmabrouk]

@Yuvakunaal assigned to you

[Yuvakunaal]

Thanks! Starting on this now.

[Yuvakunaal]

Opened #5455 — covers the archive/mtime regression test and the acceptance coverage
for the shallow (depth=1) listing and the download-all archive endpoint.

One note: _rollup_recent_entries already had unit test coverage from #5411
(TestRollupRecentEntries in test_mounts_file_ops.py), so I left that alone rather
than duplicating it — the PR only adds the two remaining gaps.