fix(ios): prevent crash on Google Maps animated gradient polyline

[lodev09]

Fixes ENG-1150.

Summary

Fixes a native iOS crash (EXC_BAD_ACCESS / GMSModelStyleBuilder couldNotInstantiate) on animated Google Maps gradient polylines, surfaced by TestFlight crash reports in the Lugg Mover app (ENG-1150).

Each display-link tick assigned _polyline.path = path then _polyline.spans = spans. Setting the new (shorter) path while the previous tick's spans were still attached makes Google Maps rebuild span models for segments that no longer exist → throws in GMSModelStyleBuilder. The empty-path branch (headDist <= tailDist) was the worst case: it shrank the path to empty but left stale spans.

The fix preserves the gradient rather than falling back to a solid stroke:

• Clear _polyline.spans = nil before swapping to a shorter/empty path, then reassign spans once the new path is set, so path and spans are never inconsistent.
• Guard the zero-segment case (clear spans, set path, return).
• Drop duplicate adjacent coordinates while building the path (zero-length segments also fail model-style instantiation).
• Apply the same hardening to the non-animated update path; createGradientSpans now keys off the deduped path's actual segment count.

Also adds an example smoke test: the gradient route is sliced to the path ahead of the moving truck, so coordinates mutate mid-animation — mirroring the live-route scenario from the crash.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Test Plan

• Ran the example on iOS (Google provider) with the truck-driven gradient polyline; gradient renders and animates with no crash.
• Note: the original crash reproduces under live GPS on device; the fix removes the path/spans inconsistency window identified in the symbolicated backtrace.

Screenshots / Videos

N/A

Checklist

• I tested on iOS
• I tested on Android
• I tested on Web
• I updated the documentation (if needed)