Fix phpstan/phpstan#14252: Enum case outside of enum should be reported

[phpstan-bot]

Summary

PHPStan did not report an error when an enum case declaration appeared inside a regular class or interface (instead of an enum). PHP itself produces a fatal error "Case can only be used in enums" for such code, but PHPStan silently accepted it.

Changes

• Added src/Rules/EnumCases/EnumCaseOutsideEnumRule.php — a new rule that checks if Node\Stmt\EnumCase appears inside a non-enum class and reports "Enum case can only be used in enums."
• Added tests/PHPStan/Rules/EnumCases/EnumCaseOutsideEnumRuleTest.php — test case for the new rule
• Added tests/PHPStan/Rules/EnumCases/data/bug-14252.php — test data with enum case in class, interface, and enum (enum case in enum should not error)

Root cause

There was no rule checking whether EnumCase AST nodes appear in the correct context. The parser (nikic/php-parser) parses case Active; as Node\Stmt\EnumCase regardless of whether it's inside an enum or a class/interface. PHPStan had rules for validating enum case attributes and sanity checks within enums, but nothing to catch an enum case declaration outside of an enum.

Test

The regression test verifies that:

• case Active; inside a class reports an error on line 9
• case Active; inside an interface reports an error on line 14
• case Active; inside an enum does NOT report an error (line 19)

Fixes phpstan/phpstan#14252