Add failing tests for ETag, HEAD requests, and conditional requests

TDD: Write tests first before implementation for:
- ETag generation and consistency in service layer
- HEAD request support (headers only, no body)
- Conditional requests with If-None-Match header (304 responses)

internal/imgcache/service_test.go

@@ -405,3 +405,111 @@ func TestService_Get_ContextCancellation(t *testing.T) {
 		t.Error("Get() expected error for cancelled context")
 	}
 }
+
+func TestService_Get_ReturnsETag(t *testing.T) {
+	svc, fixtures := SetupTestService(t)
+	ctx := context.Background()
+
+	req := &ImageRequest{
+		SourceHost: fixtures.GoodHost,
+		SourcePath: "/images/photo.jpg",
+		Size:       Size{Width: 50, Height: 50},
+		Format:     FormatJPEG,
+		Quality:    85,
+		FitMode:    FitCover,
+	}
+
+	resp, err := svc.Get(ctx, req)
+	if err != nil {
+		t.Fatalf("Get() error = %v", err)
+	}
+	defer resp.Content.Close()
+
+	// ETag should be set
+	if resp.ETag == "" {
+		t.Error("ETag should be set in response")
+	}
+
+	// ETag should be properly quoted
+	if len(resp.ETag) < 2 || resp.ETag[0] != '"' || resp.ETag[len(resp.ETag)-1] != '"' {
+		t.Errorf("ETag should be quoted, got %q", resp.ETag)
+	}
+}
+
+func TestService_Get_ETagConsistency(t *testing.T) {
+	svc, fixtures := SetupTestService(t)
+	ctx := context.Background()
+
+	req := &ImageRequest{
+		SourceHost: fixtures.GoodHost,
+		SourcePath: "/images/photo.jpg",
+		Size:       Size{Width: 50, Height: 50},
+		Format:     FormatJPEG,
+		Quality:    85,
+		FitMode:    FitCover,
+	}
+
+	// First request
+	resp1, err := svc.Get(ctx, req)
+	if err != nil {
+		t.Fatalf("Get() first request error = %v", err)
+	}
+	etag1 := resp1.ETag
+	resp1.Content.Close()
+
+	// Second request (from cache)
+	resp2, err := svc.Get(ctx, req)
+	if err != nil {
+		t.Fatalf("Get() second request error = %v", err)
+	}
+	etag2 := resp2.ETag
+	resp2.Content.Close()
+
+	// ETags should be identical for the same content
+	if etag1 != etag2 {
+		t.Errorf("ETags should match: %q != %q", etag1, etag2)
+	}
+}
+
+func TestService_Get_DifferentETagsForDifferentContent(t *testing.T) {
+	svc, fixtures := SetupTestService(t)
+	ctx := context.Background()
+
+	// Request same image at different sizes - should get different ETags
+	req1 := &ImageRequest{
+		SourceHost: fixtures.GoodHost,
+		SourcePath: "/images/photo.jpg",
+		Size:       Size{Width: 25, Height: 25},
+		Format:     FormatJPEG,
+		Quality:    85,
+		FitMode:    FitCover,
+	}
+
+	req2 := &ImageRequest{
+		SourceHost: fixtures.GoodHost,
+		SourcePath: "/images/photo.jpg",
+		Size:       Size{Width: 50, Height: 50},
+		Format:     FormatJPEG,
+		Quality:    85,
+		FitMode:    FitCover,
+	}
+
+	resp1, err := svc.Get(ctx, req1)
+	if err != nil {
+		t.Fatalf("Get() first request error = %v", err)
+	}
+	etag1 := resp1.ETag
+	resp1.Content.Close()
+
+	resp2, err := svc.Get(ctx, req2)
+	if err != nil {
+		t.Fatalf("Get() second request error = %v", err)
+	}
+	etag2 := resp2.ETag
+	resp2.Content.Close()
+
+	// ETags should be different for different content
+	if etag1 == etag2 {
+		t.Error("ETags should differ for different sizes")
+	}
+}