Detect config drift in cached tile providers and replace stale instances

When a user edits a tile type's URL template, max zoom, or API key
without changing IDs, the cached DeflockTileProvider would keep the old
frozen config. Now _getOrCreateProvider() computes a config fingerprint
and replaces the provider when drift is detected.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/services/provider_tile_cache_manager.dart

@@ -37,8 +37,11 @@ class ProviderTileCacheManager {
     required ServicePolicy policy,
     int? maxCacheBytes,
   }) {
-    assert(_baseCacheDir != null,
-        'ProviderTileCacheManager.init() must be called before getOrCreate()');
+    if (_baseCacheDir == null) {
+      throw StateError(
+        'ProviderTileCacheManager.init() must be called before getOrCreate()',
+      );
+    }
 
     final key = '$providerId/$tileTypeId';
     if (_stores.containsKey(key)) return _stores[key]!;