Refactor: use dict comprehension in _get_endpoint_prefix_to_name_mappings

## Summary

\`_get_endpoint_prefix_to_name_mappings\` in \`awscli/customizations/configure/addmodel.py\` (lines ~21-28) builds a dict manually with an empty-dict-then-loop-assignment pattern, where the loop body does nothing but a single key/value assignment.

## Proposed change

Replace with a dict comprehension:
\`\`\`python
return {
session.get_service_model(service_name).endpoint_prefix: service_name
for service_name in session.get_available_services()
}
\`\`\`
Pure refactor, no behavior change.

Open awscli/customizations/configure/addmodel.py and inspect _get_endpoint_prefix_to_name_mappings around lines 21-28. Compare the current manual loop with the proposed comprehension and verify that the endpoint-prefix-to-service-name mapping remains unchanged. Done means the function uses the requested comprehension without changing behavior.

Written by the indexing model from the issue text.