return 404 instead of 500 for missing agent config name

[ruanyl]

Description

1. Return HTTP 404 instead of 500 for missing agent config name
2. Extract error handling logic to a reusable function
3. For 5xx errors, instead of returning 500, now returning their original error code

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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.96%. Comparing base (581a7ca) to head (c0c5fc5).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #384      +/-   ##
==========================================
- Coverage   87.46%   84.96%   -2.51%     
==========================================
  Files          65       72       +7     
  Lines        1891     2068     +177     
  Branches      473      502      +29     
==========================================
+ Hits         1654     1757     +103     
- Misses        236      310      +74     
  Partials        1        1              

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