fix(llm): fix nil error when identity_interface error

[t-yuki]

Summary

Misconfigured ai-proxy causes unexpected nil error, instead of usual error, due to calling missing function.
It is occurred when bedrock model provider is configured without aws_region configuration.
In this PR, it's fixed by calling proper function.

Checklist

• The Pull Request has tests
  • no test exists for bedrock configuration
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #13666

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[fffonion]

Thank you! Nice catch!