[TypeDeclaration] Add ClosureReturnTypeFromAssertInstanceOfRector

[TomasVotruba]

Adds ClosureReturnTypeFromAssertInstanceOfRector — adds a return type to a closure inside a PHPUnit TestCase when the returned value is narrowed by assertInstanceOf().

This is the critical case ClosureReturnTypeRector cannot cover: the closure body asserts the type, so the return is known. Plain closures without assertInstanceOf() are left to ClosureReturnTypeRector.

use PHPUnit\Framework\TestCase;

final class SomeTest extends TestCase
{
    public function test()
    {
        // before
        $callback = function (object $object) {
            $this->assertInstanceOf(SomeType::class, $object);
            return $object;
        };

        // after
        $callback = function (object $object): SomeType {
            $this->assertInstanceOf(SomeType::class, $object);
            return $object;
        };
    }
}

Scope

• Closure only (arrow functions cannot hold an assertInstanceOf() statement before the return)
• Gated on enclosing class ->is('PHPUnit\Framework\TestCase')
• Requires assertInstanceOf() ($this-> or static) in the closure body; skipped otherwise
• Return type inferred via ReturnTypeInferer (PHPStan phpunit type-specifier narrows the asserted variable)
• Registered in TypeDeclarationLevel

Fixtures: assertInstanceOf-narrowed return (transforms), no-assertInstanceOf (skip), outside test case (skip).