fix(ai-proxy): internal server error on path parse failure

[tysoekong]

Summary

Fixes "Unexpected error" when using self-hosted Llama2 or Mistral models. Path parsing was incomplete, resulting in Kong trying to gsub a nil table reference.

Checklist

• The Pull Request has tests
• 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

#12869

[tysoekong]

Can make 3.7.1 instead if necessary, came from community reported error #12869

[team-gateway-bot]

Successfully created backport PR for release/3.7.x:

• [backport -> release/3.7.x] fix(ai-proxy): internal server error on path parse failure #12999

[team-gateway-bot]

Successfully created cherry-pick PR for master:

• https://github.com/kong/kong-ee/pull/9063