Restore per-request credential override in AwsExecutionContextBuilder

[S-Saranya1]

Motivation and Context

Per-request credential overrides via AwsRequestOverrideConfiguration.credentialsProvider() were silently ignored when using new core with old service clients. The override was only applied in AuthSchemeResolutionStage, which exits early when AUTH_SCHEME_OPTIONS_RESOLVER is not set - an attribute only new codegen produces.

Modifications

Restored application of per-request credential overrides in AwsExecutionContextBuilder.putAuthSchemeResolutionAttributes() so that IDENTITY_PROVIDERS reflects the override at context construction time, before the old generated AuthSchemeInterceptor reads it.

Testing

• Added unit test in AwsExecutionContextBuilderTest verifying that IDENTITY_PROVIDERS contains the per-request credential override even when AUTH_SCHEME_OPTIONS_RESOLVER is not set.
• Also verified locally with old-client-version-compatibility-test using DynamoDB 2.40.12 (post-SRA, pre-pipeline-stage) to confirm the fix works end-to-end with an actual old service client and new core.

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