feat(ai_completion): support both /response and /chat/completion OpenAI endpoints

[savonarola]

Fixes EMQX-14603

Release version: 6.0.0

Summary

PR Checklist

• For internal contributor: there is a jira ticket to track this change
• The changes are covered with new or existing tests
• Change log for changes visible by users has been added to changes/ee/(feat|perf|fix|breaking)-<PR-id>.en.md files
• Schema changes are backward compatible or intentionally breaking (describe the changes and the reasoning in the summary)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.51%. Comparing base (3db32c2) to head (d9d57bb).
⚠️ Report is 5 commits behind head on release-60.

Files with missing lines	Patch %	Lines
...completions/emqx_ai_completion_openai_response.erl	72.72%	3 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           release-60   #15724      +/-   ##
==============================================
+ Coverage       84.48%   84.51%   +0.03%     
==============================================
  Files            1135     1136       +1     
  Lines           79189    79209      +20     
==============================================
+ Hits            66901    66943      +42     
+ Misses          12288    12266      -22     

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[emqxqa]

TestExecution