Tracking issue from closed PR #768.

The TOCTOU fix in PR #766 introduced error handling branches that reduced ssl-bump.ts branch coverage from 100% to 81.25%, failing the coverage regression check.

Changes needed

• Add tests for EEXIST error handling (file already exists - should be silently caught)
• Add tests for non-EEXIST errors (permission denied, etc. - should be re-thrown)
• Add test for non-Error throws in generateSessionCa (string rejection handling)
• Fix ESLint rule name: prefer-promise-reject-with-error → @typescript-eslint/prefer-promise-reject-errors

Target: ssl-bump.ts branch coverage from 81.25% → 100%

Original PR: #768