chore: add sonnet-4-6 mapping, config defaults, and CI improvements

- Add claude-sonnet-4-6 to default Antigravity model mapping
- Add antigravity_extra_retries default value in config
- Add cache-dependency-path to CI setup-go for faster builds
- Simplify vitest config to avoid vite plugin compatibility issues

backend/internal/service/antigravity_model_mapping_test.go

@@ -76,6 +76,12 @@ func TestAntigravityGatewayService_GetMappedModel(t *testing.T) {
 		},
 
 		// 3. 默认映射中的透传（映射到自己）
+		{
+			name:           "默认映射透传 - claude-sonnet-4-6",
+			requestedModel: "claude-sonnet-4-6",
+			accountMapping: nil,
+			expected:       "claude-sonnet-4-6",
+		},
 		{
 			name:           "默认映射透传 - claude-sonnet-4-5",
 			requestedModel: "claude-sonnet-4-5",