[2.x] fix: widen Extend\SearchDriver class-string types to SearcherInterface

[imorland]

Summary

The docblock annotations on Extend\SearchDriver::addSearcher() / addFilter() / replaceFilter() / setFulltext() / addMutator() were typed class-string<AbstractSearcher>, but the prose right next to them already said:

This searcher must implement \Flarum\Search\SearcherInterface.
Or extend \Flarum\Search\Database\AbstractSearcher if using the default driver.

The type annotation was strictly narrower than the prose. Custom non-DB search drivers — the whole reason SearcherInterface exists separately from AbstractSearcher — would trip phpstan with a false positive on every single addSearcher() / addFilter() call in their extend.php.

This PR widens the annotations to class-string<SearcherInterface>, which is what the runtime actually checks against (line 50/66/etc. just assign the string into an array) and what the docblocks have always described. AbstractSearcher extends/implements SearcherInterface, so this is a strictly looser type — no breakage for the default driver, and custom-driver extensions get a phpstan-clean run.

Also drops the now-unused use Flarum\Search\Database\AbstractSearcher; import.

Changes

• framework/core/src/Extend/SearchDriver.php: 5 docblock annotations widened, 1 import removed, 1 import added.

Test plan

• No behavioural change — all 5 affected methods just assign their string param into an array. The widening only affects what static analysis tools accept.
• Existing tests pass.
• Extensions that pass AbstractSearcher subclasses still type-check (covariance — narrower is fine).
• Extensions that pass plain SearcherInterface implementers now type-check (this is the bug being fixed).