Line3: increase test coverage (backport #813)

[mergify]

🦟 Bug fix

Improve test coverage of the Line3 class, similar to #807

Summary

This increases test coverage based on suggestions from Gemini and uses static_cast<T> where needed to fix windows C4244 compiler warnings.

Summary of test coverage improvements

1. Default Constructor: Tested Line3() initialization.
2. Set Overload: Added tests for Set(const Vector3<T>&, const Vector3<T>&).
3. Intersect Edge Cases: Added tests for parallel non-overlapping line segments, parallel overlapping segments, and non-intersecting skew lines.
4. Template Types: Added test cases for Line3i (int) and Line3f (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 #813 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 578203f has failed:

On branch mergify/bp/gz-math8/pr-813
Your branch is up to date with 'origin/gz-math8'.

You are currently cherry-picking commit 578203f.
  (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:   include/gz/math/Line3.hh
	modified:   src/Line3_TEST.cc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/python_pybind11/test/Line3_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

[scpeters]

I'd like to generate backports from #821 since it has the fixed commit attribution and doesn't need the DCO bot warning to be manually ignored