Line2: increase test coverage (backport #807)

[mergify]

🦟 Bug fix

Improve test coverage of Line2 class

Summary

This increases test coverage based on suggestions from Gemini. After observing some windows C4244 compiler warnings, I fixed them in 2a719e4:

• Fix warnings in Set(double, double, double, double) by wrapping with static_cast<T>
• Fix warning in Length() by using the simpler implementation from Line3.hh wrapped with static_cast<T>:

gz-math/include/gz/math/Line3.hh

Lines 135 to 138 in 396f5a5

	public: T Length() const
	{
	return this->pts[0].Distance(this->pts[1]);
	}

Summary of test coverage improvements

1. Set with Vector2 parameters: Verified setting start/end points using Vector2 instances.
2. CrossProduct: Added explicit tests for both Line2 and Vector2 cross products. Expect CrossProduct of parallel lines is 0.
3. OnSegment and Within: Tested points on segment, collinear points outside bounds, non-collinear points within bounding box, and endpoints.
4. Intersect Branch Coverage:
   • Parallel collinear non-overlapping line segments.
   • Collinear lines where only the second endpoint lies on the segment.
   • Line extensions that intersect outside X or Y bounding ranges.
   • Intersect overload taking no output point.
5. Operators: Expanded operator== and operator!= self-comparison checks.
6. Template Types: Added test cases for Line2i (int) and Line2f (float).

Backport Policy

• This is safe to backport to the following versions:
  • Jetty
  • Ionic
  • Harmonic
  • Fortress
• This should not be backported
• I am not sure
• Other (fill in yourself)

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
• While waiting for a review on your PR, please help review another open pull request to support the maintainers
• Was GenAI used to generate this PR? If so, make sure to add "Assisted-by" to your commits. (See this policy for more info.)

Assisted-by: Gemini 3.6 Flash

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by and Generated-by messages.

Backports: If this is a backport, please use Rebase and Merge instead.

---

This is an automatic backport of pull request #807 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 268039c has failed:

On branch mergify/bp/ign-math6/pr-807
Your branch is up to date with 'origin/ign-math6'.

You are currently cherry-picking commit 268039c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   src/Line2_TEST.cc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   include/gz/math/Line2.hh
	both modified:   src/python_pybind11/test/Line2_TEST.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.85%. Comparing base (5b2522a) to head (bfae060).
⚠️ Report is 14 commits behind head on ign-math6.

Additional details and impacted files

@@              Coverage Diff              @@
##           ign-math6     #829      +/-   ##
=============================================
- Coverage      99.38%   97.85%   -1.54%     
=============================================
  Files             75       75              
  Lines           7029     7582     +553     
=============================================
+ Hits            6986     7419     +433     
- Misses            43      163     +120     

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