Fix: PHPStan errors in Datagrid and Filter classes

[radimvaculik]

Summary

• Use Nette\Application\UI\Presenter for lookupPath() in Datagrid::getSortableParentPath() — Nette\Application\IPresenter is just a marker interface (not an IComponent), so it doesn't satisfy class-string<IComponent>.
• Remove redundant instanceof Form checks in FilterSelect and FilterMultiSelect — Container::lookup(Form::class) already guarantees the return type via its conditional return type.

Test plan

• make phpstan → [OK] No errors

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.86%. Comparing base (e5faff0) to head (7f579a3).

Files with missing lines	Patch %	Lines
src/Datagrid.php	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1285      +/-   ##
==========================================
+ Coverage   46.79%   46.86%   +0.06%     
==========================================
  Files          53       53              
  Lines        2729     2725       -4     
==========================================
  Hits         1277     1277              
+ Misses       1452     1448       -4     

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