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).