fix(search): end unified search pagination and order results stably

[AlyNotMe]

Closes #9067

What

• Validate the Unified Search cursor and limit before any database access
• Request limit + 1 rows to detect a next page; return complete() when
  there is none and paginated() only when more rows remain
• Apply the SQL offset directly (setFirstResult/setMaxResults)
  instead of deriving a fractional page number
• Add id as a tie-breaker after created_at so equal timestamps keep
  a stable order. The shared query builder is untouched, so other file
  lists keep their current sorting

Tests

• Unit: cursor/limit validation (the mapper is never called on invalid
  input), limit + 1 request, empty/partial/exact-limit/next-page cases,
  and the extra row is never returned
• Database: stable order with equal timestamps, consecutive pages without
  duplicates or gaps, offset not a multiple of the limit, limit respected
• Ran composer test:unit, composer test:integration (new test),
  composer cs:check and composer psalm locally

[YvesCesar]

@AlyNotMe

Thanks for the PR! I reviewed and tested it locally on MySQL:

• tests/php/Unit/Search/FileSearchProviderTest.php: 21 tests passing
• tests/php/Integration/Db/SignRequestMapperSearchPaginationTest.php: 5 tests passing
• Running the same tests against the previous code fails, so they really cover the fix.

Cursor validation, the limit + 1 next-page detection and the f.id tie-breaker all look correct to me. No changes requested from my side.

[AlyNotMe]

Thanks for the review, @YvesCesar, and thanks @vitormattos for assigning me!

About the failing PHPUnit integration jobs: they are all the same
PageControllerTest ArgumentCountError (PageController::__construct():
Argument #10 $validationEffectivePolicyService not passed). That constructor
argument was added on main by 20d806a and PageControllerTest was not
updated, so it also fails without this PR. Happy to open a small separate
PR to fix that test if you'd like.

One more thing: I'm a Node.js backend developer looking for my first
developer job, and the companies I'm talking to ask me for real open source
contributions in PHP. I enjoyed working on this one, and I'd be glad to
keep contributing to LibreSign. Is there anything I should know to become a
regular contributor, and are there other good first issues you would
recommend I take next? I'll comment on an issue before starting, as the
contributing guide asks.

Thanks again for your time!