fix: don't apply initialCameraFit on a zero-sized camera

[nisenbeck]

Fixes #2222 - see that issue for the full bug report, reproduction steps and root-cause analysis.

Fix

_applyInitialCameraFit could apply initialCameraFit against a zero-sized camera (constraints.maxWidth/maxHeight == 0) and mark it as permanently applied, because _parentConstraintsAreSet can return true in exactly that situation (its MediaQuery-based fallback is satisfied almost immediately, independently of whether this widget's own layout has actually settled).

Added an explicit, additional guard so the fit is only applied - and only latched as applied - once the constraints are genuinely non-zero in both dimensions:

if (!_initialCameraFitApplied &&
    widget.options.initialCameraFit != null &&
    _parentConstraintsAreSet(context, constraints) &&
    constraints.maxWidth > 0 &&
    constraints.maxHeight > 0) {
  _initialCameraFitApplied = true;
  _mapController.fitCamera(widget.options.initialCameraFit!);
}

I deliberately left _parentConstraintsAreSet itself untouched, rather than rewriting its heuristic - there's no history explaining why the MediaQuery fallback was added, so I didn't want to risk changing behaviour I can't fully verify. This fix instead directly prevents the harmful outcome (latching against a degenerate size) regardless of what _parentConstraintsAreSet decides. If the real constraints never arrive (the widget is genuinely given zero space forever), the fit simply never applies, which is a strictly better outcome than silently locking in a degenerate zoom.

Testing

Added test/map/zero_size_camera_fit_test.dart with two cases (there was no existing test coverage for initialCameraFit at all):

• A real FlutterMap mounted inside a widget that reports Size.zero for one frame before revealing its real size: asserts the resulting camera.zoom matches a direct fit against the real size (previously it settled on 0.0, the degenerate zoomed-out result).
• The same fit with the real size available from the very first frame (no race), to confirm the already-working case is unaffected by this change.

Full existing test suite passes locally with no regressions.

Manually verified on a physical Android device using the reproduction linked in the issue, pointed at this branch via a dependency_overrides: path: entry.

AI disclosure

Per CONTRIBUTING.md: I used AI assistance (Claude Code) to help diagnose the root cause and draft this fix and its tests. I've reviewed the change, verified it against the project's lint rules and test suite, and manually confirmed the fix resolves the issue on a physical device.

[JaffaKetchup]

LGTM! Thanks again :)