objc: Fix VolumeType enum name clash on MacOS #29262
### Pull Request Readiness Checklist
- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
- [ ] The feature is well documented and sample code can be built with the project CMake
**Problem**
When the ObjC wrapper is generated, the global enum `cv::VolumeType` from the ptcloud module is emitted as `typedef NS_ENUM(int, VolumeType)`. macOS defines `typedef OSType VolumeType` in CoreServices, which leads to a typedef-redefinition error during the framework build.
**Fix**
`add_enum()` in `gen_objc.py` now handles namespace-global enums by falling back to a module-level lookup (`self.Module`) when the enum has no enclosing class. A new `gen_dict.json` entry maps `VolumeType` -> `PtcloudVolumeType`, and the import generation automatically uses the renamed enum.
Fixes#29260
Haris shakeel