Upgrade PHPStan to 2.x and raise analysis level from 6 to 10

- Bump phpstan/phpstan ^2.0 and szepeviktor/phpstan-wordpress ^2.0
- Move the analysis level into phpstan.neon (single source) and raise it to 10
- Add Val, a runtime coercion helper that narrows untyped WordPress boundary
  values (wpdb rows, REST params, superglobals, options) with explicit checks
  instead of blind casts, plus unit tests
- Type value-object fromRow() params as stdClass (what wpdb returns) and map
  columns through Val so unexpected shapes degrade safely
- Use %i identifier placeholders for table names in all wpdb::prepare() calls
  so every query string is a literal and identifiers are escaped by WordPress;
  raises the minimum WordPress version to 6.2 where %i was introduced
- Guard wpdb::prepare() null result before wpdb::query() in updateTax()
- Fix nullable get_permalink()/strtotime() handling, list types at REST and
  capability call sites, dead null-coalescing on checked superglobals, and
  narrow get_users() results before mapping
- Register Val method names with the ValidatedSanitizedInput sniff so it
  validates the real sanitizer around each superglobal read
- Update repository unit tests for the %i placeholder arguments

Co-Authored-By: Claude Fable 5 <[email protected]>

src/Payment/BillingMethodResolver.php

@@ -3,6 +3,8 @@ declare(strict_types=1);
 
 namespace Unsupervised\Schedular\Payment;
 
+use Unsupervised\Schedular\Val;
+
 /**
  * Resolves the billing method for a student: a per-student override if set,
  * otherwise the studio default — card when Stripe is configured, e-transfer when
@@ -15,7 +17,7 @@ class BillingMethodResolver {
 	public function __construct( private StudioSettings $settings ) {}
 
 	public function resolve( int $studentId ): string {
-		$override = (string) get_user_meta( $studentId, self::META_METHOD, true );
+		$override = Val::string( get_user_meta( $studentId, self::META_METHOD, true ) );
 		if ( in_array( $override, Payment::VALID_METHODS, true ) ) {
 			return $override;
 		}