Make Arrays::flatten type-correct

[RubenKluft]

Make Arrays::flatten generic
Adds Phpstan extension to make this possible, as phpstan does not support recursive type definition, an extension is required

[Copilot]

Pull request overview

This PR introduces a PHPStan dynamic static method return type extension to provide more precise (recursive) generic type inference for DR\Utils\Arrays::flatten(), since PHPStan can’t express the desired recursive generic natively.

Changes:

• Adds ArraysFlattenReturnExtension to compute flattened leaf item types for Arrays::flatten() calls.
• Updates Arrays::flatten() PHPDoc to document a generic template and reference the extension for precise typing.
• Adds unit + integration PHPStan type inference tests and registers the extension via extension.neon (with baseline updates).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/Unit/PHPStan/Extension/ArraysFlattenReturnExtensionTest.php	Adds a basic unit test for the new return-type extension.
tests/Integration/PHPStan/ArraysFlattenReturnExtensionTest.php	Adds PHPStan type inference integration test harness for flatten assertions.
tests/Integration/PHPStan/data/ArraysFlattenAssertions.php	Adds type assertions that validate inferred return types for various flatten inputs.
src/PHPStan/Extension/ArraysFlattenReturnExtension.php	Implements dynamic return type inference for Arrays::flatten().
src/Arrays.php	Updates flatten() PHPDoc to a generic template and references the extension.
phpstan-baseline.neon	Baselines new PHPStan API deprecation notices introduced by the extension.
extension.neon	Registers the new PHPStan return type extension as a service.

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