[PHPUnit120] Use Stub FQN in @var docblock

[TomasVotruba]

Both MockObjectVarToStubRector (#689) and BareVarToStubIntersectionRector (#690) wrote the short Stub name into the @var docblock, which requires a use import to resolve.

This always emits the FQN \PHPUnit\Framework\MockObject\Stub instead, so no import check is needed and the type resolves regardless of imports.

 /**
- * @var FieldModel|Stub
+ * @var FieldModel|\PHPUnit\Framework\MockObject\Stub
  */
 private \PHPUnit\Framework\MockObject\Stub $leadFieldModel;

Fixtures and code samples updated. Tests, ECS, PHPStan, Rector all pass.