Bump PHPStan rule level to 4

[westonruter]

This PR is based off of #7619, see additional commits: justlevine/wordpress-develop@feat/phpstan...westonruter:wordpress-develop:bump/phpstan-level-4

Trac ticket: https://core.trac.wordpress.org/ticket/64680

By increasing the PHPStan rule level to 4, the following issues will begin to be detected for new code introduced to core:

1. possibly undefined variables, unknown magic methods and properties on classes with __call and __get
2. unknown methods checked on all expressions (not just $this), validating PHPDocs
3. return types, types assigned to properties
4. basic dead code checking - always false instanceof and other type checks, dead else branches, unreachable code after return; etc.

Existing errors for these levels are ignored by a baseline which has been updated to include 641 errors:

Count	PHPStan Identifier
56	while.alwaysTrue
55	deadCode.unreachable
53	property.notFound
52	property.nonObject
36	isset.property
34	return.type
29	staticClassAccess.privateMethod
23	assign.propertyType
19	property.defaultValue
16	class.notFound
14	method.unused
13	method.nonObject
13	function.alreadyNarrowedType
12	booleanAnd.rightAlwaysTrue
10	return.unusedType
10	notIdentical.alwaysTrue
9	empty.variable
9	empty.property
9	arguments.count
8	varTag.noVariable
8	greaterOrEqual.alwaysTrue
8	constant.notFound
8	booleanNot.alwaysTrue
7	property.private
7	parameterByRef.type
7	method.childParameterType
6	phpDoc.parseError
6	if.alwaysFalse
5	property.phpDocType
5	parameter.defaultValue
5	nullCoalesce.property
5	method.notFound
5	isset.variable
5	isset.offset
5	if.alwaysTrue
4	variable.undefined
4	parameter.notFound
4	identical.alwaysFalse
4	booleanAnd.leftAlwaysTrue
3	return.missing
3	property.protected
3	offsetAccess.notFound
3	booleanNot.alwaysFalse
3	booleanAnd.alwaysFalse
3	binaryOp.invalid
2	ternary.alwaysFalse
2	return.empty
2	property.unusedType
2	parameterByRef.unusedType
2	offsetAccess.nonOffsetAccessible
2	nullCoalesce.offset
2	identical.alwaysTrue
2	function.impossibleType
2	empty.offset
2	booleanOr.alwaysTrue
1	while.alwaysFalse
1	ternary.alwaysTrue
1	smallerOrEqual.alwaysTrue
1	property.onlyWritten
1	parameter.unresolvableType
1	parameter.phpDocType
1	offsetAssign.valueType
1	instanceof.alwaysTrue
1	foreach.nonIterable
1	encapsedStringPart.nonString
1	constructor.unusedParameter
1	booleanOr.rightAlwaysTrue
1	booleanOr.alwaysFalse
1	booleanAnd.rightAlwaysFalse
1	booleanAnd.alwaysTrue

Use of AI Tools

Summarizing the ignored errors in the baseline.

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