Feat/handler error handling and docs

[retr0-kernel]

Description

• Improved error handling and validation in API handlers (application.go, audit.go)
• Updated Swagger comments to document all error responses (422, 500, etc.)

Related Issue

Closes #4094

Motivation and Context

These changes improve API robustness, make error responses more predictable, and help new contributors understand and follow best practices.

Screenshots (if appropriate)

N/A

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 14.28571% with 36 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 33.17%. Comparing base (e23bd66) to head (a8f7bd8). :warning: Report is 168 commits behind head on main.

Files with missing lines	Patch %	Lines
manager/handlers/application.go	9.67%	22 Missing and 6 partials :warning:
manager/handlers/audit.go	27.27%	6 Missing and 2 partials :warning:

:x: Your project status has failed because the head coverage (33.17%) is below the target coverage (34.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4095      +/-   ##
==========================================
- Coverage   34.36%   33.17%   -1.19%     
==========================================
  Files         346      346              
  Lines       40489    40625     +136     
==========================================
- Hits        13913    13478     -435     
- Misses      25660    26248     +588     
+ Partials      916      899      -17     

Flag	Coverage Δ
unittests	33.17% <14.28%> (-1.19%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
manager/handlers/audit.go	54.16% <27.27%> (-17.27%)	:arrow_down:
manager/handlers/application.go	53.26% <9.67%> (-16.44%)	:arrow_down:

... and 27 files with indirect coverage changes

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[retr0-kernel]

@gaius-qi I guess we need to re-run the jobs not sure what are the errors for the second job (which is a required one)