feat: implement snapshot-integrated custom LRO error parsing (POC 3)

[nnicolee]

Overview

This PR implements POC 3 for custom LRO error details propagation. In this design, we encapsulate the custom error parsing concern directly inside HttpJsonOperationSnapshot during its instantiation lifecycle.

This approach keeps ProtoOperationTransformers completely clean and aligned with the gRPC implementation, avoiding the need for multiple overloads or passing custom error parser objects down to the response transformer.

Changes

1. GAX Core (gax-httpjson)

• HttpJsonOperationSnapshot.java: Added .setErrorParser(HttpJsonLroErrorParser) to HttpJsonOperationSnapshot.Builder. If configured, build() invokes the parser on the response object to populate errorDetails and errorMessage on the built snapshot instance.
• ProtoOperationTransformers.java: Reverted the ResponseTransformer to a single clean implementation. It no longer accepts HttpJsonLroErrorParser and instead directly reads operationSnapshot.getErrorDetails() and operationSnapshot.getErrorMessage().

2. Generator (gapic-generator-java)

• HttpJsonServiceStubClassComposer.java: Conditionally generates .setErrorParser(new ComputeLroErrorParser()) on the HttpJsonOperationSnapshot builder for long-running compute APIs (ignoring the v1small integration test package to maintain golden safety).
• RetrySettingsComposer.java: Reverted the POC 2 changes that registered the error parser inside the ResponseTransformer settings factory.

Verification

• Run integration test suite: bazelisk test //sdk-platform-java/test/integration:compute -> PASSED
• Run generator unit tests: mvn test -pl :gapic-generator-java -> PASSED

[gemini-code-assist]

Code Review

This pull request introduces support for propagating custom Long-Running Operation (LRO) error details across both gRPC and HTTP/JSON transports. It adds the HttpJsonLroErrorParser interface, implements a ComputeLroErrorParser for Google Cloud Compute, and updates OperationSnapshot and ProtoOperationTransformers to carry ErrorDetails. However, the package matching logic in HttpJsonServiceStubClassComposer incorrectly targets com.google.cloud.compute.v1small as well, which will cause compilation failures in the generated golden files because ComputeLroErrorParser is package-private and unavailable in that package. This should be resolved by explicitly excluding v1small from the generator's condition.

[gemini-code-assist]

The package check service.pakkage().startsWith("com.google.cloud.compute.v1") evaluates to true for "com.google.cloud.compute.v1small". This causes the generator to incorrectly inject .setErrorParser(new ComputeLroErrorParser()) into the v1small golden files (as seen in HttpJsonAddressesStub.java and HttpJsonRegionOperationsStub.java).

Since ComputeLroErrorParser is a hand-written class in com.google.cloud.compute.v1.stub and is package-private, it is not present or accessible in com.google.cloud.compute.v1small.stub, which will cause compilation failures when building the goldens.

To fix this and properly ignore the v1small package as intended, we should explicitly exclude it in the condition.

Suggested change

	if (service.pakkage().startsWith("com.google.cloud.compute.v1")) {
	if (service.pakkage().startsWith("com.google.cloud.compute.v1")
	&& !service.pakkage().startsWith("com.google.cloud.compute.v1small")) {

[sonarqubecloud]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions
40.7% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud