OSV feed Ubuntu: Validate artifacts

[sharon-fdm]

Closes #42199 — adds OSV artifact validation to the existing cmd/cve/validate/main.go script. No changes needed in the vulnerabilities repo since generate-cve.yml already runs this validator after OSV artifact generation.

Note: AI-generated draft PR. Please review carefully.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.85%. Comparing base (4f9fe1d) to head (a9a8dd7).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/cve/validate/main.go	0.00%	56 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #43359      +/-   ##
==========================================
- Coverage   66.89%   66.85%   -0.04%     
==========================================
  Files        2588     2588              
  Lines      207563   207619      +56     
  Branches     9320     9320              
==========================================
- Hits       138840   138807      -33     
- Misses      56096    56177      +81     
- Partials    12627    12635       +8     

Flag	Coverage Δ
backend	68.63% <0.00%> (-0.04%)	⬇️

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