Fix phpstan/phpstan#8270: False positive on array item modification

[phpstan-bot]

Summary

When modifying a sub-key of an array element at a non-constant offset (e.g., $list[$k]['test'] = true), PHPStan incorrectly concluded that ALL array items had the modified value. This caused false positives like "If condition is always true" when iterating over the array and checking the modified key.

Changes

• Added shouldUnionExistingItemType() method in src/Analyser/ExprHandler/AssignHandler.php that checks whether a composed value changes existing constant-array key values
• Modified produceArrayDimFetchAssignValueToWrite() to force unionValues=true at the outermost dimension when the composed value has conflicting key values with the existing item type
• Updated test expectations in tests/PHPStan/Rules/Arrays/data/bug-11679.php and tests/PHPStan/Rules/Arrays/data/slevomat-foreach-array-key-exists-bug.php to match the more correct union behavior
• Added regression test in tests/PHPStan/Analyser/nsrt/bug-8270.php

Root cause

In produceArrayDimFetchAssignValueToWrite(), nested array dimension assignments like $list[$k]['test'] = true are processed bottom-up. The inner dimension correctly computes the modified element value (e.g., array{test: true, value: int}). However, at the outer dimension, setOffsetValueType() was called with unionValues=false, which replaced the entire item type with the composed value. Since the offset key is non-constant (could match any element), this incorrectly assumed ALL elements had the new value.

The fix detects when the composed value changes existing key values (e.g., test: false → test: true) by comparing key-by-key. When a conflict is found, it forces unionValues=true so the item type becomes the union of old and new (e.g., test: bool), correctly representing that some elements retain their original values.

The check is limited to the outermost dimension to avoid issues with deeply nested array building patterns (like bug-1388).

Test

Added tests/PHPStan/Analyser/nsrt/bug-8270.php with two test functions:

1. A simple case with a @var-annotated non-empty-list and constant offset $list[0]['test'] = true
2. A realistic case matching the original bug report: building a list in a loop, narrowing with array_key_first(), modifying one element, then iterating

Both verify that after modifying one element's sub-key, the foreach item type correctly shows bool (not true) for the modified key.

Fixes phpstan/phpstan#8270