[2.x] fix: add missing getHidden() to Log\Context\Repository stub

[imorland]

Summary

Laravel 13's queue handler calls getHidden() on the log context repository when releasing unique-job locks after a ModelNotFoundException (CallQueuedHandler::ensureUniqueJobLockIsReleasedViaContext).

Flarum's stub at framework/core/src/Log/Context/Repository.php implemented addHidden, forgetHidden, and allHidden but not getHidden. So any queued job whose subject (post, discussion, etc.) was deleted between dispatch and execution turned a benign ModelNotFoundException into a cluster of fatal Error: Call to undefined method Flarum\Log\Context\Repository::getHidden() — one per pending job for that subject.

Add getHidden() mirroring get() for the hidden array. With nothing writing to the hidden context in a fresh worker, the call returns null and Laravel's handler silently skips lock cleanup — the intended no-op-on-empty path.

Fixes #4611

Changes

• framework/core/src/Log/Context/Repository.php — add getHidden(string $key, mixed $default = null): mixed.
• New framework/core/tests/unit/Log/Context/RepositoryTest.php — 4 unit tests covering getHidden return value, default fallback, add/get/forget round-trip, and isolation between public and hidden arrays.

Test plan

• Unit tests pass (4 tests, 9 assertions).
• Verified the new tests catch the original bug — reverting the fix produces 4× Call to undefined method errors, exactly the runtime error users hit.
• Manual: dispatch a queued job that touches a soon-to-be-deleted model (e.g. mention a user, then delete the post immediately); confirm the worker logs the ModelNotFoundException quietly without the fatal cascade.

Notes

• The stub still doesn't implement many other Laravel Repository methods (pullHidden, hasHidden, pushHidden, …) — adding those is out of scope here. This PR fixes the specific path that's currently fataling in production.