Fix fill-extrusion heights for features with a non-numeric building_id

[enersis-pst]

Fixes #13712.

The bug

A fill-extrusion layer whose features carry a building_id string (a UUID, in the
reporter's case) renders with wrong heights over terrain. Removing the property makes
the heights correct again.

Root cause

building_id groups the parts of one building so that they share a centroid. It was
read with Number(feature.properties['building_id']) and never validated, so any
non-numeric value became NaN.

NaN is where this turns into a visible bug: Map and Set keys compare with
SameValueZero, under which NaN equals itself. So the buildingGroups map collapsed
every feature with a non-numeric building_id into a single group.
_finalizeBuildingGroups() then gave each of them one shared centroid and the bounding
box merged across all of them. Since fill-extrusion-height-alignment defaults to
flat, that centroid is exactly what decides the roof elevation over terrain — so each
building got a roof height derived from the average of all unrelated buildings.

The same NaN pattern was latent in BuildingBucket, where a NaN id in the
disabledBuildings set would have disabled every other building along with it.

The fix

resolveBuildingId() in src/data/building_id.ts reports unusable values as absent
instead of returning NaN. Both buckets fall back to their existing "no building_id"
path, which for fill extrusions means grouping by feature id — the behaviour the reporter
already confirmed as correct. A single warnOnce makes the cause visible.

Numeric strings such as "42" keep working, which was the intent of a60f5c6
(building_id values are compared against feature ids, and those are always numbers).

Note this does not add native support for string building_id values, one of the options
raised in the issue. buildingId is packed into a Float64Array and into a
Set<number> used for conflation, so a string would need a stable hash with a collision
risk — a feature decision rather than part of this bug fix.

Tests

• test/unit/data/building_id.test.ts — the resolver's return values.
• test/unit/data/fill_extrusion_bucket.test.ts — bucket-level behaviour: two features
  with distinct non-numeric ids keep separate centroids and bounding boxes, features
  sharing a numeric id still get merged, and non-numeric ids fall back to the feature id.
  Without the fix both UUID buildings share the centroid 26006,26006.
• test/integration/render-tests/fill-extrusion-terrain/flat-roof-non-numeric-building-id/
  — two boxes on a slope with ~38m of terrain between them. Verified in both directions:
  it passes with the fix and fails without it (without it the downhill box is stretched
  and the uphill box is squashed).

Launch Checklist

• Make sure the PR title is descriptive and preferably reflects the change from the user's perspective.
• Add additional detail and context in the PR description (with screenshots/videos if there are visual changes).
• Manually test the debug page.
• Write tests for all new functionality and make sure the CI checks pass.
• Document any changes to public APIs. — no public API change
• Post benchmark scores if the change could affect performance. — parsing path only, no measurable effect
• Tag @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes. — no style spec change
• Tag @mapbox/gl-native if this PR includes shader changes or needs a native port. — no shader change, but the same Number() conversion likely exists on the native side
• Tag @mapbox/gl-native if this PR disables any test because it also needs to be disabled on their side. — no test disabled
• Create a ticket for gl-native ... — not applicable

[github-actions]

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