fix: sample elevation lookups from the rendered terrain surface (#8213)

* fix: sample elevation lookups from the rendered terrain surface

Where a location is covered by a rendered tile whose DEM has loaded,
getElevationForLngLat now samples the same surface the raycast hits,
so project() round-trips unproject() exactly. The covering-tile
traversal remains the fallback while tiles load and outside the
rendered coverage.

* test: drop the terrain fixture parameters with only one value in use

* test: wire each two-zoom terrain test's source tiles inline

* test: inline the two-zoom terrain setup into each test

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Co-authored-by: Harel M <harel.mazor@gmail.com>