Narrow parent constant array type when type-checking an array offset

[phpstan-bot]

Summary

• When is_string($arr['key']) (or similar type-check functions) narrows an array offset's type, also propagate a sureNotType on the parent array variable. This removes constant array members whose offset value is fully covered by the narrowing, preventing false positives when BooleanAnd's intersectWith drops entries for different expression keys.
• Extend the coalesce simplification in createForExpr's false context so that is_string($x ?? null) in the falsey branch simplifies to narrowing $x (since string is non-null, the ?? null fallback is irrelevant).
• Also fixes the same false positive in bug-9004.

Test plan

• tests/PHPStan/Rules/Arrays/OffsetAccessAssignmentRuleTest::testBug14566 — regression test with both isset && is_string and is_string($x ?? null) patterns
• tests/PHPStan/Rules/Arrays/OffsetAccessAssignmentRuleTest::testBug9004 — existing test updated (was asserting buggy behavior)
• Full test suite passes (11,981 tests, 79,553 assertions)
• PHPStan self-analysis passes (make phpstan)
• Coding standards pass (make cs-fix)

Closes phpstan/phpstan#14566

[VincentLanglet]

Explains why

/**
 * @param array{}|array{hi: 'hello'}|array{hi: array{0: 42, 1?: 42}} $test
 */
function foo(array $test): void {
	if (!isset($test['hi'])) {
		return;
	}
	if (\is_string($test['hi'])) {
		return;
	}
	$test['hi'][] = 42;
}

/**
 * @param array{}|array{hi: 'hello'}|array{hi: array{0: 42, 1?: 42}} $test
 */
function foo2(array $test): void {
	if (!\is_array($test['hi'] ?? null)) {
		return;
	}
	$test['hi'][] = 42;
}

already works fine

[phpstan-bot]

Done. Added the two equivalent patterns from VincentLanglet's review as additional test cases (foo3 with split if statements, foo4 with is_array + ?? null) to document that these already worked before the fix. All tests (11,986) and make phpstan pass. Pushed to the branch.