test(files): make FolderTest multi-storage pagination coverage deterministic

[joshtrichards]

• Resolves: #

Summary

This fixes flaky multi-storage search pagination coverage in FolderTest and adds clearer separation between default-order and explicit-order assertions.

Changes:

• applied minor clarity/readability refactoring to Folder::search() (originally thought this may be a production code matter)
• split the existing sub-storage pagination coverage into:
  • default-order tests that verify safe invariants only
  • explicit-order tests that verify exact path ordering when the sort is deterministic
• removed reliance on implicit cross-storage fileid ordering in test expectations
• extracted shared fixture/query setup helpers to reduce duplication and make the tests easier to follow

Why:

The previous test mixed two different concerns:

• pagination behavior with no explicit ordering
• pagination behavior with explicit deterministic ordering

For multi-storage search results, asserting an exact default order across sub-storages was brittle and could fail depending on how file IDs were assigned across caches. The updated tests keep coverage for the default case, but only assert properties that are safe under unspecified ordering, while preserving strict sequence assertions for explicitly ordered queries.

• fixes a flaky CI/test failure unrelated to the PR under test
• preserves useful pagination coverage
• makes the intent of the test suite clearer
• avoids over-asserting ordering semantics that the fixture does not guarantee

Fixes failures like https://github.com/nextcloud/server/actions/runs/28053658919/job/83050710177?pr=61555

There was 1 failure:

1) Test\Files\Node\FolderTest::testSearchSubStoragesLimitOffset with data set #1 (0, 5, ['/bar/foo/foo1', '/bar/foo/foo2', '/bar/foo/foo3', '/bar/foo/foo4', '/bar/foo/sub1/foo5'], [])
Failed asserting that two arrays are equal.
--- Expected
+++ Actual
@@ @@
     1 => '/bar/foo/foo2'
     2 => '/bar/foo/foo3'
     3 => '/bar/foo/foo4'
-    4 => '/bar/foo/sub1/foo5'
+    4 => '/bar/foo/sub2/foo7'
 )

/home/runner/actions-runner/_work/server/server/tests/lib/Files/Node/FolderTest.php:1061

TODO

• Double-check
• Backport to 34 + 33?

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