Merge pull request #28621 from mvanhorn:osc/28528-fix-houghcircles-return-type

imgproc: fix HoughCircles Python return type to allow None #28621

### Pull Request Readiness Checklist

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- [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
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

### Description

Fixes https://github.com/opencv/opencv/issues/28528

`HoughCircles` returns `None` when no circles are found, but the auto-generated Python type stub declares the return type as `MatLike` without `None`. This causes type checkers (mypy/pyright) to miss potential `None` dereferences.

**Fix:** Add `HoughCircles` to `NODES_TO_REFINE` in `api_refinement.py` using the existing `make_optional_none_return` helper - the same pattern already used for `imread` and `imdecode`.

This contribution was developed with AI assistance (Claude Code).

modules/python/src2/typing_stubs_generation/api_refinement.py

@@ -457,6 +457,9 @@ NODES_TO_REFINE = {
     SymbolName(("cv", "fisheye"), (), "initUndistortRectifyMap"): make_optional_arg("D"),
     SymbolName(("cv", ), (), "imread"): make_optional_none_return,
     SymbolName(("cv", ), (), "imdecode"): make_optional_none_return,
+    SymbolName(("cv", ), (), "HoughCircles"): make_optional_none_return,
+    SymbolName(("cv", ), (), "HoughLines"): make_optional_none_return,
+    SymbolName(("cv", ), (), "HoughLinesP"): make_optional_none_return,
     # Fix for issue #28534: inRange should accept Scalar for lowerb and upperb
     SymbolName(("cv", ), (), "inRange"): make_matlike_or_scalar_arg("lowerb", "upperb"),
 }