Improve coverage for list workflow handlers

[Shaddoll]

What changed?

• Move list workflow handlers to a separate file
• Refactor list workflow handlers
• Improve unit test coverage for list workflow handlers

Why?
Improve code quality

How did you test it?
unit tests

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

Attention: Patch coverage is 65.43536% with 131 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@115a0da). Learn more about missing BASE report.
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
service/frontend/api/list_workflow_handlers.go	51.66%	102 Missing and 29 partials ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
service/frontend/api/handler.go	71.20% <ø> (ø)
service/frontend/api/request_validator.go	100.00% <100.00%> (ø)
service/frontend/api/list_workflow_handlers.go	51.66% <51.66%> (ø)

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