test: remove Oxlint suppressions

[GuillaumeRx]

Explanation

The repository-wide Oxlint suppression file hides test lint violations in apply-release-plan and create-package. This PR removes the suppression file and fixes the underlying test code instead.

The fixes move repeated changed-file validation into a shared test helper, replace conditional expectations with unconditional assertions, use every instead of a conditional reducer, and move the mocked file lookup into a helper.

References

N/A

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them