Fix GPXTrackSegment.length_2d return annotation

[saitakarcesme]

GPXTrackSegment.length_2d() delegates to geo.length_2d(), which always returns a number, including zero for empty or single-point segments. Its Optional[float] annotation nevertheless prevents callers from using that result as a float without a None check. Change it to float, matching the implementation and the track/GPX length methods.

Fixes #286.

The regression test uses explicitly typed assignments for empty, single-point, and two-point segments, and checks the returned distances. All three assignments fail mypy before the annotation fix and pass afterward.

Validation on Python 3.13.5:

• All 150 unit tests pass with lxml 6.1.3, and again with the standard-library XML backend.
• gpxinfo runs successfully on all GPX fixtures; git diff --check passes.
• Full strict mypy 2.3.1 reports 19 pre-existing diagnostics. Comparing before/after output shows only the three regression diagnostics removed and no added diagnostics.

Prepared and tested using OpenAI Codex; no human review is claimed.

[tkrajina]

Merged, thanks