fix: prevent RangeError in Proj4Crs.zoom for out-of-range scales

[AlexLaroche]

Summary

Fixes a RangeError thrown by Proj4Crs.zoom when the requested scale is finer than the deepest level defined in the CRS's resolutions list. Closes #1223 (which has documented this exact crash since 2022).

Root cause

In lib/src/geo/crs.dart:

final downScale = _closestElement(_scales, scale);     // returns _scales.last when scale > all entries
final downZoom = _scales.indexOf(downScale);           // = _scales.length - 1
// ...
final nextZoom = downZoom + 1;                         // = _scales.length
final nextScale = _scales[nextZoom];                   // RangeError

When the caller asks for a scale beyond the largest defined level, _closestElement returns _scales.last, so downZoom is the last index. The interpolation block then indexes _scales[downZoom + 1] and throws.

The most common path that hits this is FitBounds._getBoundsZoom, which calls camera.getScaleZoom(scale) before clamping the result to maxZoom (camera_fit.dart:162 vs :176). So setting maxZoom on MapOptions / CameraFit.bounds does not protect against the crash — it crashes inside Proj4Crs.zoom before the clamp can run. This is also why issue #1223's reproduction is fitBounds with a custom CRS.

Fix

When downZoom == _scales.length - 1 and the requested scale is strictly larger than downScale, return double.infinity instead of falling through to the out-of-bounds index access.

This mirrors the existing lower-bound behavior in the same function: when the requested scale is smaller than every entry in _scales, the function already returns double.negativeInfinity. The fix makes the upper bound symmetric.

Callers like FitBounds._getBoundsZoom already .clamp(min, max) the returned zoom, so +∞ becomes maxZoom and the camera settles correctly. The whole frame no longer throws.

Why double.infinity rather than clamping to _scales.length - 1?

• Symmetric with the existing double.negativeInfinity for the lower bound.
• Lets the caller decide the clamp policy — FitBounds already has its own maxZoom that may be lower than the deepest CRS level.
• Unambiguous sentinel: distinguishes "scale exactly at deepest level" from "scale beyond all defined levels".

Happy to switch to a finite clamp if reviewers prefer.

Test plan

• New regression test in test/geo/crs_test.dart covers exact match, interpolation, lower-bound -∞, and upper-bound +∞.
• Verified the new +∞ test reproduces the original RangeError at crs.dart:356:30 when the fix is reverted.
• flutter test test/geo/ test/flutter_map_test.dart — all 20 tests pass.
• flutter analyze lib/src/geo/crs.dart test/geo/crs_test.dart — no issues.
• dart format clean on changed files.