test(files): expand and improve quota wrapper coverage

[joshtrichards]

• Resolves: #n/a

Summary

Fixes some existing test bugs for OC\Files\Storage\Wrapper\Quota and expands targeted PHPUnit coverage focusing on previously-untested branches and quota- boundary behavior.

Notes

• The changes are test-only and motivated by some documentation work + possible production code changes (and just generally good test hygiene!); no production code is modified in this PR.
• Fixes are in the first commit; bulk of new tests are in second commit; third commit adds a new test specifically for external storage quota inclusion handling.

New coverage:

• Exact quota boundary behavior (file_put_contents/writeStream reject exact-fit writes due to current strict < comparison)
• quotaCallback invocation and caching (getQuota() only calls the callback once)
• Unlimited (FileInfo::SPACE_UNLIMITED) and negative quota values delegate to wrapped storage
• enableQuota(false) disables enforcement
• cache/ and uploads/ path prefixes bypass quota
• .part files bypass quota when opened for writing
• copyFromStorage/moveFromStorage correctly reject and leave source/target untouched when quota is exceeded
• Sized writeStream() return values and exact-limit rejection (previously the return value wasn't asserted)
• include_external_storage quota accounting correctly reflects mounted external storage in free_space()

Fix:

• Removed an invalid assertion in testNoMkdirQuotaZero() (assertFalse($instance->mkdir('files'))). This was passing for the wrong reason: getLimitedStorage() already creates files/ on the underlying storage before the test runs. In addition, shouldApplyQuota() only applies quota to paths matching files/ (with trailing slash), so bare files was never quota-scoped to begin with.

TODO

• Make sure all tests, particularly the integration ones pass reliably enough in CI run(s)
• Backport? (I vote at least to v34 to have some better coverage at least thru it's lifecycle)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI