test: add comprehensive coverage for TOCTOU fix error paths

[Mossaka]

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

[github-actions]

👋 Hello! I noticed this issue is closely related to:

• fix: eliminate TOCTOU race conditions in ssl-bump.ts #838 - "fix: eliminate TOCTOU race conditions in ssl-bump.ts"

Both issues were created from the same closed PRs (#766 and #768) and are part of the same TOCTOU fix work:

• fix: eliminate TOCTOU race conditions in ssl-bump.ts #838 tracks the implementation of the atomic file creation fix
• test: add comprehensive coverage for TOCTOU fix error paths #837 (this issue) tracks adding comprehensive test coverage for the error paths

These appear to be companion tasks that should be addressed together. The implementation in #838 should be completed before the test coverage in this issue can be fully implemented.

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