test(gax): Error Logging Integration Test

[westarle]

Add an automated integration test (ITActionableErrorsLogging) to ensure end-to-end functionality of the Actionable Errors logging framework against a livegapic-showcase server.

• Integration Test Implementation: Added ITActionableErrorsLogging to verify that LoggingTracer correctly extracts and emits google.rpc.ErrorInfo context (reason, domain, and metadata) as structured DEBUG logs during failed attempts.
• Reflection-Free Execution: Completely eliminated Java reflection and explicit Logback casting. The test now relies on the standard TestLogger and TestServiceProvider implementation provided by gax-testlib.
• Build Configuration: Updated gax and gapic-showcase pom.xml files to correctly expose META-INF/services/** and include the test exclusively when the -PenvVarTest profile is
  activated.
• Transport Coverage: Includes tests for both gRPC and HTTP/JSON transports (using a MockHttpTransport to simulate the correct JSON error payload for REST).

[gemini-code-assist]

Code Review

This pull request introduces integration tests for actionable error logging in both gRPC and HTTP/JSON transports, along with necessary build configuration updates in the GAX and Showcase modules. The changes include a new Maven profile for running environment-specific tests and the addition of the ITActionableErrorsLogging test suite. Review feedback focuses on narrowing the scope of included service providers in the POM to avoid bundling unnecessary files, centralizing dependency version management, removing unused imports and variables, and ensuring proper resource cleanup by calling awaitTermination after closing clients in tests.