Add no-failure output mode matrix

[Will-thom]

Summary

• add a table-driven no-failure output matrix for text, JSON, stdout, and file output modes
• assert exit code 0 remains stable across no-failure output paths
• verify JSON no-failure output keeps findingCount 0 and findings as an empty array

Validation

• go test ./cmd/prmaven -run TestNoFailureOutputModeMatrix -v
• git diff --check
• .github/scripts/check-public-metadata.sh
• go test ./...

Closes #72

[github-actions]

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