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/UpdatePlantUseCaseTests.swift

@@ -254,7 +254,7 @@ final class UpdatePlantUseCaseTests: XCTestCase {
         isFavorite: Bool = false,
         notes: String? = nil,
         customName: String? = nil,
-        location: String? = nil
+        roomID: UUID? = nil
     ) -> Plant {
         Plant(
             id: id,
@@ -263,9 +263,10 @@ final class UpdatePlantUseCaseTests: XCTestCase {
             family: family,
             genus: genus,
             identificationSource: .onDeviceML,
+            notes: notes,
             isFavorite: isFavorite,
             customName: customName,
-            location: location
+            roomID: roomID
         )
     }