cache: add QueryHash and CachedEngineResponse type

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

internal/cache/cache_test.go

@@ -75,3 +75,35 @@ func TestNew_NopWithoutAddress(t *testing.T) {
 }
 
 func strPtr(s string) *string { return &s }
+
+func TestQueryHash(t *testing.T) {
+	// Same params should produce same hash
+	hash1 := QueryHash("golang", 1, 0, "en", "")
+	hash2 := QueryHash("golang", 1, 0, "en", "")
+	if hash1 != hash2 {
+		t.Errorf("QueryHash: same params should produce same hash, got %s != %s", hash1, hash2)
+	}
+
+	// Different query should produce different hash
+	hash3 := QueryHash("rust", 1, 0, "en", "")
+	if hash1 == hash3 {
+		t.Errorf("QueryHash: different queries should produce different hash")
+	}
+
+	// Different pageno should produce different hash
+	hash4 := QueryHash("golang", 2, 0, "en", "")
+	if hash1 == hash4 {
+		t.Errorf("QueryHash: different pageno should produce different hash")
+	}
+
+	// time_range should affect hash
+	hash5 := QueryHash("golang", 1, 0, "en", "day")
+	if hash1 == hash5 {
+		t.Errorf("QueryHash: different time_range should produce different hash")
+	}
+
+	// Hash should be 16 characters (truncated SHA-256)
+	if len(hash1) != 16 {
+		t.Errorf("QueryHash: expected 16 char hash, got %d", len(hash1))
+	}
+}