Refactor test data setup for relation-based fixtures and align PHPUnit docs

[spawnia]

• Added or updated tests
• Documented user facing changes
• Updated CHANGELOG.md

Changes

This PR aligns test fixture setup with CONTRIBUTING.md#test-data-setup:

• Refactored targeted integration tests to prefer relations over direct FK writes
• Replaced array-fill model setup in tests (for example ->make(['id' => 1])) with property assignment style
• Removed literal ID coupling in refactored WhereConditionsDirectiveTest methods by using created model instances/keys
• Updated docs/master/testing/phpunit.md examples to use /** @lang GraphQL */ plus nowdoc (<<<'GRAPHQL'), and property assignment style in setup examples

Touched files:

• tests/Integration/Schema/Types/UnionTest.php
• tests/Integration/Bind/BindDirectiveTest.php
• tests/Integration/WhereConditions/WhereHasConditionsDirectiveTest.php
• tests/Integration/Schema/Directives/AllDirectiveTest.php
• tests/Integration/OrderBy/OrderByDirectiveDBTest.php
• tests/Integration/WhereConditions/WhereConditionsDirectiveTest.php
• CONTRIBUTING.md
• docs/master/testing/phpunit.md
• .ai/AGENTS.md

Validation run locally:

• make fix
• make test (passes; existing notices/skips/incomplete remain)
• make stan

Breaking changes

None.