[google_maps_flutter_web] Gracefully bypass HeatmapLayer when unsupported by Maps JS SDK

[jeffkwoh]

This Pull Request resolves the critical runtime TypeError / NoSuchMethodError crash in the google_maps_flutter_web package caused by the Google Maps JavaScript API version 3.65 (May 2026) officially deprecating and removing the HeatmapLayer constructor (google.maps.visualization.HeatmapLayer).
Because google_maps_flutter_web historically attempted to instantiate this class directly on the web platform when adding a heatmap, the deprecation broke the web package examples and caused shapes_test.dart and shape_test.dart to crash, blocking the monorepo's web_platform_tests CI builders.

Proposed Resolution (Graceful Feature Detection)

Since Heatmap is a core shared platform interface API, we preserve its signature but implement dynamic, strongly-typed feature-detection at the web platform level:

1. Phased Feature Detection (dom_window_extension.dart):
   Added safe, nullable @JS() interop properties on web.Window and JSObject to traverse namespaces sequentially:
   nullableGoogle $\rightarrow$ nullableMaps $\rightarrow$ nullableVisualization $\rightarrow$ nullableHeatmapLayer.
   This allows isHeatmapSupported() to safely check the loaded Maps JS SDK capabilities without throwing any nested property-access exceptions on undefined JS objects.
2. Graceful Bypassing & Deduplication (heatmaps.dart):
   Shielded the addHeatmaps, changeHeatmaps, and removeHeatmaps methods in HeatmapsController to return early when isHeatmapSupported() is false. If unsupported, it logs a standard console warning via Flutter's debugPrint at most once per controller lifecycle using a private _warningLogged boolean flag, making all heatmap operations safe no-ops rather than runtime crashes.
3. Dynamic Test Skipping (shape_test.dart & shapes_test.dart):
   Updated both integration test files to import the internal dom_window_extension.dart directly using implementation imports (// ignore: implementation_imports) to avoid code duplication. Both HeatmapController and HeatmapsController test groups are dynamically skipped via skip: !isHeatmapSupported() when running in browser environments loaded with SDK version 3.65+.

Fixes: flutter/flutter#187076

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