Fix LineSegmentDetector segfault caused by C++/C# calling convention mismatch

[shimat]

Summary

Fixes #1876.

LineSegmentDetector.Create() returned an instance with a null RawPtr in 4.13.0.20260427, causing a segfault on Detect().

Root cause

PR #1846 updated the C# P/Invoke declarations for
imgproc_Ptr_LineSegmentDetector_get and imgproc_Ptr_LineSegmentDetector_delete to the new ExceptionStatus + out-parameter convention, but the corresponding C++ implementations in imgproc_LineSegmentDetector.h were left in the old
direct-return style. The calling-convention mismatch caused rawPtr to be populated with garbage/null, leading to the segfault.

Fix

Update the two C++ functions to match the convention already used everywhere else in the codebase (e.g. bgsegm.h, calib3d_StereoMatcher.h):

• Return ExceptionStatus instead of void / raw pointer
• Wrap the body with BEGIN_WRAP / END_WRAP
• Pass the result via an output pointer parameter

Also adds LineSegmentDetectorTest covering the exact repro case from the issue.

Summary by CodeRabbit

• Tests

  • Added comprehensive test suite for LineSegmentDetector functionality, covering object creation, line detection operations, and various output handling scenarios.

• Refactor

  • Updated LineSegmentDetector API interfaces with enhanced error handling and resource management patterns for improved stability and consistency.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

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Plan: Pro

Run ID: 985b3b67-d7eb-4907-b8db-8aa6c697424c

📥 Commits

Reviewing files that changed from the base of the PR and between d721631 and 75b4baa.

📒 Files selected for processing (2)

• src/OpenCvSharpExtern/imgproc_LineSegmentDetector.h
• test/OpenCvSharp.Tests/imgproc/LineSegmentDetectorTest.cs

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📝 Walkthrough

Walkthrough

This PR updates the LineSegmentDetector C++ wrapper functions to return ExceptionStatus with proper exception wrapping instead of returning void or raw pointers directly. Two functional tests validate the detector creation and line detection across both detection method variants.

Changes

LineSegmentDetector wrapper and validation

Layer / File(s)	Summary
LineSegmentDetector wrapper exception handling src/OpenCvSharpExtern/imgproc_LineSegmentDetector.h	Wrapper functions imgproc_Ptr_LineSegmentDetector_delete and imgproc_Ptr_LineSegmentDetector_get now return ExceptionStatus and wrap operations in BEGIN_WRAP/END_WRAP. The get function takes an output pointer parameter (cv::LineSegmentDetector **returnValue) instead of returning the pointer directly.
LineSegmentDetector functional tests test/OpenCvSharp.Tests/imgproc/LineSegmentDetectorTest.cs	New test class with CreateAndDetect verifying detector instantiation and line detection with return value, and DetectOutputArray testing the output parameter variant of detection. Both tests create synthetic images with drawn lines and validate non-empty results.

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: fixing a segfault in LineSegmentDetector caused by a C++/C# calling convention mismatch.
Linked Issues check	✅ Passed	The changes address issue #1876 by fixing the C++ function signatures in imgproc_LineSegmentDetector.h to match the ExceptionStatus pattern and output parameter convention, restoring correct native-pointer handling.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the LineSegmentDetector segfault issue. Header modifications fix the calling convention mismatch, and test additions validate the exact scenario described in issue #1876.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/competent-shannon-5acdac

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