ci: fix Detect Changes job failing on push to main

[yan-3005]

Add checkout step before dorny/paths-filter@v3 in the changes job. For push events, paths-filter runs git branch --show-current locally which fails without a checkout; pull_request events use the GitHub API and are unaffected.

Describe your changes:

Fixes

Type of change:

• Bug fix
• Improvement
• New feature
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation

Checklist:

• I have read the CONTRIBUTING document.
• My PR title is Fixes <issue-number>: <short explanation>
• I have commented on my code, particularly in hard-to-understand areas.
• For JSON Schema changes: I updated the migration scripts or explained why it is not needed.

[gitar-bot]

Code Review ✅ Approved

Fixes the Detect Changes job that was failing on push to main by updating the CI configuration. No issues found.

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[Copilot]

Pull request overview

Fixes the CI workflow’s “Detect Changes” job on push to main by ensuring the repository is checked out before running dorny/paths-filter@v3, which relies on local git state for non-PR events.

Changes:

• Added an actions/checkout@v4 step to the changes job (skipped for workflow_dispatch).
• Ensures dorny/paths-filter@v3 can run successfully on push events by having a local git repository available.

[github-actions]

OpenMetadata Service New-Code Coverage

✅ No changed production Java files under openmetadata-service/src/main/java. Coverage gate skipped.