Remove resolveClientEndpointProvider from AwsDefaultClientBuilder

[S-Saranya1]

Motivation and Context

AwsDefaultClientBuilder.resolveClientEndpointProvider() was triggering GeneratedServiceMetadataProvider initialization (~350ms) during every client creation. Although registered via lazyOptionIfAbsent as a fallback for older clients, AttributeMap.build() eagerly resolves all lazy values — causing the fallback to execute even when the generated builder already set CLIENT_ENDPOINT_PROVIDER. All generated clients have set it themselves since 2024, making this fallback dead code with a significant performance cost.

Additionally, AwsClientEndpointProvider.build() is the method that calls ServiceMetadata.of() and triggers the expensive initialization. After our changes across PRs #7094, #7105, #7110, #7168, no code path in client creation calls it anymore. Deprecating it signals to not use and use resolveFromOverrides() + service EndpointProvider instead, and prevents accidental re-introduction of the performance issue.

Modifications

• Removed resolveClientEndpointProvider() method and its lazyOptionIfAbsent registration from AwsDefaultClientBuilder.finalizeAwsConfiguration()
• Removed unused AwsClientEndpointProvider import and DEFAULT_ENDPOINT_PROTOCOL constant
• Added null validation in resolveEndpoint() and resolveEndpointOverridden() with clear error message for custom clients that don't set CLIENT_ENDPOINT_PROVIDER
• Deprecated AwsClientEndpointProvider.build() — points users to resolveFromOverrides() + service EndpointProvider instead
• Updated TestClientBuilder and TestAsyncClientBuilder in tests to set CLIENT_ENDPOINT_PROVIDER in finalizeServiceConfiguration(), mirroring what real generated builders do

Testing

• All 32 existing tests in DefaultAwsClientBuilderTest pass
• Tests updated to set CLIENT_ENDPOINT_PROVIDER directly (same pattern as generated builders)

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
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• 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