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).

[scpeters]

there is an issue with the DCO due to the mangled name and email in https://github.com/gazebosim/gz-math/commit/578203f7bde5091a15b04f597c64ddf179929bf0.patch

From: Steve Peters ahcorde@gmail.com

I will amend and force-push

[scpeters]

I will amend and force-push

done in 0e36e71

confirm by inspecting https://github.com/gazebosim/gz-math/commit/0e36e71067b80334175d0fc068b1286593e68aca.patch

From: Steve Peters scpeters@intrinsic.ai