[2.x] fix(phpstan): recognize conditional model casts - #4920

Changes

• inspect the callable arrays returned by all Conditional extender methods
• support both arrow functions and closure return bodies without executing them
• add end-to-end PHPStan fixtures for extension, setting, and generic conditions

Why

Conditional moved from direct extender arrays to callable|string, but the
PHPStan resolver still looked only for a direct array on
whenExtensionEnabled(). Valid model casts nested in the current callable API
were therefore reported as undefined properties.

Fixes #4808

Testing

• Failing-first fixture reproduced property.notFound on unchanged 2.x
• Full vendor/bin/phpstan analyse -c phpstan.neon --no-progress --error-format=table --memory-limit=2G
• PHP syntax checks for the resolver and both fixtures
• composer validate --no-check-publish (valid; existing dependency-constraint warnings only)