[6.x] Fix Modify::__toString() throwing TypeError for non-string scalar values

[VerburgtJimmy]

Closes #14671.

Statamic\Modifiers\Modify::__toString() was calling method_exists($this->value, '__toString') without first checking that $this->value is an object or string. PHP 8+ made method_exists() strictly typed, so it throws a TypeError on ints, floats, bools, null, and arrays. That means something like (string) Statamic::modify(0)->formatMoney() blows up any time the chain returns an int, which is exactly what Cargo's formatMoney modifier does for a zero price.

Fix

• Cast scalars (int/float/bool/string) and null to string directly.
• Only call method_exists() when the value is actually an object.
• The fallback ModifierException now uses get_debug_type() instead of get_class(), so the error path doesn't itself throw when the value is an array or scalar.

Tests

Added to tests/Modifiers/FluentModifyTest.php:

• Data provider covering 0, positive/negative ints, a float, true/false, null, and plain strings.
• Modify::value(0)->add(5) → "5" (the example from Statamic::modify() throws TypeError when value is integer 0 #14671).
• Arrays still throw ModifierException with the expected message.