Report implicit mixed inside compound types as too wide at all rule levels

[phpstan-bot]

Closes phpstan/phpstan#8681

Summary

Since 1.9.5, PHPStan stopped reporting that a bare array (implicit array<mixed, mixed>) is too wide for a declared @return array<string, string>. The root cause was in RuleLevelHelper::transformCommonType(): an early return checked !$this->checkExplicitMixed && !$this->checkImplicitMixed and short-circuited the entire transformation, preventing implicit mixed inside compound types from being converted to StrictMixedType.

The fix:

• Narrows the early return to only fire when the type itself is a top-level MixedType (not a compound type containing mixed)
• Adds a $isTopLevel tracker so that implicit mixed at non-top-level positions (array value types, array key types, callable return types, generic type parameters) is converted to StrictMixedType even when checkImplicitMixed is false
• Guards against converting TemplateMixedType, which should remain as-is

This restores the pre-1.9.5 behavior where array is not silently accepted where array<string, string> is expected, regardless of rule level.

Scope of changes

The fix applies to all call sites of RuleLevelHelper::accepts(), meaning it affects:

• Return type checking (methods, functions, closures) — the original issue
• Property assignment checking — array assigned to array<string, string> property
• Parameter type checking — array passed where array<string, string> parameter expected

New errors are now correctly reported for:

• array where array<K, V> is expected (both key and value mismatch)
• iterable where iterable<K, V> is expected
• Closure(): mixed where callable(): int is expected (implicit mixed return type)
• Generic<mixed> where Generic<Specific> is expected (when mixed is implicit)

Test changes

• Added regression test data files for methods, functions, and property assignments (bug-8681.php)
• Updated 20 existing test expectations to reflect newly-detected type mismatches
• Updated PHPStan baseline for 49 newly-detected errors in PHPStan's own codebase
• Fixed @return string[] → @return list<string> annotations on test helper methods exposed by the change