Fix flaky DynamoDB enhanced client integration tests

[S-Saranya1]

Motivation and Context

Several DynamoDB enhanced client integration tests flake in preview builds due to eventual consistency:

• SearchVectorsIntegrationTestBase (SearchHappyPathTests): Fails with No result with sort key: dot-unbounded-item because POST_MUTATION_SEARCH_RETRY_ATTEMPTS (8 retries, ~4.5s) was insufficient for vector index propagation under CI load.

• SearchVectorsIntegrationTestBase (WritePathTests): writePath_updateNonVectorFieldsOnly fails at ~0.026s because searchVectorsUntilContains checks item presence but not content, returning immediately with stale data after an update.

• QueryGSICompositeKeysIntegrationTest (Static, Immutable, Bean schemas): Various queryGsi* tests fail because waitForGsiConsistency() used a fixed 3-second Thread.sleep, which is insufficient for GSI propagation under CI load. 43 failure runs in the last 30 days.

Modifications

• Removed POST_MUTATION_SEARCH_RETRY_ATTEMPTS and consolidated into DEFAULT_SEARCH_RETRY_ATTEMPTS (15), since post-mutation searches need the same consistency window.
• Added content-aware retry in writePath_updateNonVectorFieldsOnly to wait for the updated description to propagate, not just item presence.
• Replaced fixed Thread.sleep(3000) in waitForGsiConsistency() with a poll-and-retry loop that scans gsi1 until all records are visible, with exponential backoff (500ms-2s, up to 20 attempts).

Testing

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
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• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license