fix(phpstan): improved method validity checks and reduced error handling

This commit fixes several issues in different PHP files.
- Added `method_exists` checks for `isActive` method in `activate.php`, `deactivate.php` and
`toggle.php` to prevent calls to an undefined method.
- Similar check added for `getTitle` method in `get.php`.
- Fixed sorting functionality in `search.php` by using `strcmp` instead of greater than comparison.
- Removed a large number of ignored errors in `phpstan-baseline.neon`, indicating reduced error
handling.
- Corrected the `implode` parameters order in `Discount.php` and typecasted variable to string
before using `explode` in `PriceFactor.php`.
- Updated `getTitle` & `toPriceFactor` method parameters in `Discount.php` to use union types for
compatibility with different data types.

Related: #18