Fix: Don't pass null $old_value to single-arg callable sanitizers

[AlexGStapleton]

Deprecated: intval(): Passing null to parameter #2 ($base) of type int is deprecated in wp-content/plugins/so-widgets-bundle/base/inc/fields/base.class.php on line 405

Fix deprecated intval() call when a field uses a callable sanitizer such as 'intval'. sanitize() passed $old_value as a second argument to every callable sanitizer, so a null $old_value was forwarded to intval()'s optional $base parameter, triggering a PHP 8.1+ deprecation and, when a non-null old value was present, silently converting with the wrong base.

Only pass $old_value to callables that declare a second required parameter:

• Add private sanitize_callback_accepts_old_value() using reflection (ReflectionFunction / ReflectionMethod, incl. 'Class::method' strings), defaulting to false on reflection failure.
• One-argument callables like 'intval' are now invoked with the value only; callables that genuinely require $old_value keep receiving it.
• Uses the two-argument ReflectionMethod constructor for PHP 7.0 compatibility.

Add tests/BaseFieldSanitizeTest.php covering the intval sanitizer (null and non-null old value), one-param callables, and two-required-param callables (closure, array, and static method forms). The intval test reproduces the reported deprecation against the unpatched code.

[Misplon]

Verified: new tests fail on develop (5), branch green (25). Closure, 'Class::method' and invokable branches checked. Premium CPT Builder's sanitize_reserved_post_types( $post_type, $old_value ) still receives $old_value. Also fixes a latent fatal: 1-arg internal sanitizers like 'strlen' throw ArgumentCountError on develop.

Change requested: the check is too broad.

PHP ignores extra arguments to user-defined functions. The deprecation, wrong-base and ArgumentCountError problems only occur with internal functions. But required >= 2 also strips $old_value from user callables that got it on develop:

• function ( $value, $old_value = null ) — loses $old_value
• function ( $value, ...$args ) — loses it
• 'sanitize_title_with_dashes' (Premium lightbox) — fallback behaviour changes

Fix:

return ! ( $reflection->isInternal() && $reflection->getNumberOfRequiredParameters() < 2 );

Internal + fewer than 2 required params → value only. Everything else → two args, as on develop. Closure::fromCallable( 'intval' ) reflects internal on 8.3, so it is covered too.

Tests:

1. The one-param data provider never registers the provided callables — the variadic wrapper is the sanitizer in every case. The one-param ReflectionMethod branch is untested.
2. After the fix above, assert single-arg dispatch for internal callables only. Assert user callables (one-param, optional-second, variadic) still receive both args.