test(showcase): add integration tests for exponential backoff retries using SequenceService

[nnicolee]

Description

This PR adds E2E retry integration tests verifying that client-side retry attempts adhere to the configured exponential backoff settings for both gRPC and HTTP/JSON transports.

Key Changes

• SequenceService Client configuration:
  • Adds client initializer methods to TestClientInitializer for creating the SequenceServiceClient configured with custom retry parameters and disabled jitter.
• Timing-Safe Timing assertions:
  • Uses the mock server's stateful SequenceService to record attempt timestamps on the server side, removing local CPU context-switching scheduling lag as a source of test flakiness.
  • Configures a standard error sequence ([UNAVAILABLE, UNAVAILABLE, UNAVAILABLE, OK]) and asserts that retry delays fall within the target exponential backoff timeline (with lenient safety margins).

[gemini-code-assist]

Code Review

This pull request introduces integration tests for verifying exponential backoff retries on both gRPC and HTTP/JSON clients using the SequenceServiceClient. It also adds corresponding helper methods in TestClientInitializer to initialize these clients with custom retry settings. The feedback suggests simplifying the helper methods in TestClientInitializer by configuring SequenceServiceSettings.Builder directly, which avoids intermediate stub settings and unnecessary object allocations.