bug: fitBounds behavior can be inconsistent when terrain elevation is enabled

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### mapbox-gl-js version

v3.21.0

### Browser and version

tested in Chromium Version 145.0.7632.159 (Official Build) (64-bit), and Firefox 140.0 (64-bit)

### Expected behavior

calling `fitBounds` with the same bounds, on the same map, with the same settings, should result in the same camera zoom and center.

### Actual behavior

calling `fitBounds` with the same bounds, on the same map, with the same settings, results in different camera zoom and center when terrain is set on the map.

### Link to the demonstration

https://jsbin.com/saxasovuzo/edit?html,output

### Steps to trigger the unexpected behavior

The demonstration has been programmed to call `fitBounds` twice in a row in order to trigger the unexpected behavior.

### Relevant log output

```shell
=== call 1 ===
[call 1] current: center=(0.00000000, 0.00000000) z=1.000000
[call 1] target: center=(33.38546777, -2.57608454) z=19.039717
[call 1] terrain: nw=null se=null ne=null sw=null

=== call 2 (from moveend of call 1) ===
[call 2] current: center=(33.38514890, -2.57608454) z=19.039718
[call 2] target: center=(33.38546777, -2.57608454) z=15.968305
[call 2] terrain: nw=1149.3545487659221 se=1153.7093150069609 ne=1149.1340876072286 sw=1154.144002061813
```

Start by running the linked JSBin reproduction in Chromium or Firefox and compare the two consecutive fitBounds calls with terrain enabled. Trace the fitBounds camera calculation and terrain elevation inputs; done means repeated calls with identical bounds and settings produce the same camera center and zoom.

Written by the indexing model from the issue text.