[CodeQuality] Skip DecorateWillReturnMapWithExpectsMockRector on non-test methods

[TomasVotruba]

Improves DecorateWillReturnMapWithExpectsMockRector to only decorate willReturnMap() calls inside setUp() and actual test methods.

Previously it ran in any method except setUp(). Now it skips private helpers and protected/non-test methods (e.g. prepareSomeMock(), createSomeMock()), where the expects() count is not meaningful.

• Switched node type to ClassMethod to reuse TestsNodeAnalyzer::isTestClassMethod() (handles test prefix, #[Test] attribute, @test annotation).
• Added skip fixtures for private helper and protected non-test method.