Fix test infrastructure for Rooms feature and improve testability

- Update Plant test fixtures to use roomID instead of deprecated location
- Add URLDataFetcher protocol to ImageCache for dependency injection
- Update ImageCacheTests to use protocol-based mock instead of URLSession subclass
- Add missing cancelReminders(for:plantID:) method to MockNotificationService
- Add Equatable conformance to ImageCacheError for test assertions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

PlantGuideTests/TestFixtures/Plant+TestFixtures.swift

@@ -31,7 +31,7 @@ extension Plant {
     ///   - notes: User notes. Defaults to nil.
     ///   - isFavorite: Favorite status. Defaults to false.
     ///   - customName: User-assigned name. Defaults to nil.
-    ///   - location: Plant location. Defaults to nil.
+    ///   - roomID: Room ID where plant is located. Defaults to nil.
     /// - Returns: A configured Plant instance for testing
     static func mock(
         id: UUID = UUID(),
@@ -48,7 +48,7 @@ extension Plant {
         notes: String? = nil,
         isFavorite: Bool = false,
         customName: String? = nil,
-        location: String? = nil
+        roomID: UUID? = nil
     ) -> Plant {
         Plant(
             id: id,
@@ -65,7 +65,7 @@ extension Plant {
             notes: notes,
             isFavorite: isFavorite,
             customName: customName,
-            location: location
+            roomID: roomID
         )
     }
 
@@ -206,7 +206,7 @@ extension Plant {
             notes: "Needs regular watering and indirect light",
             isFavorite: true,
             customName: "My Beautiful Monstera",
-            location: "Living room by the window"
+            roomID: nil
         )
     }
 }