Fix swagger definition of usage

[postsa]

Proposed changes

the dce-cli currently fails with

dce usage -e 1725645854 -s 1725645000
err:  json: cannot unmarshal array into Go value of type operations.GetUsageOKBody

because it's expecting a single value but the API returns an array.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (changes to code, which do not change application behavior)

Checklist

• I have filled out this PR template
• I have read the CONTRIBUTING doc
• I have added automated tests that prove my fix is effective or that my feature works
• I have added necessary documentation (README.md, inline comments, etc.)
• I have updated the CHANGELOG.md under a ## next release, with a short summary of my changes

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Further comments

[CLAassistant]

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