Optimize the response of AI agent APIs

[gaobinlong]

Description

When calling the APIs like api/assistant/summary, api/assistant/insight and other related APIs, if OpenSearch cluster returns 4xx error, we just return 500 error or 400 error directly, which is not elegant, we should optimize this by checking the status code, if it's 4xx error, we return the original response, and if it's 5xx error, return an general error message.

Issues Resolved

[List any issues this PR will resolve]

Check List

• [ ] New functionality includes testing.
  • [ ] All tests pass, including unit test, integration test.
• [ ] New functionality has user manual doc added.
• [ ] Commits are signed per the DCO using --signoff.

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[gaobinlong]

@ruanyl @SuZhou-Joe @Hailong-am @xluo-aws please help to review this PR, thanks!