require timestamping leaf certificate to be an end-entity

[dxbjavid]

verifyLeafCert and VerifyCertChain enforce that a timestamping leaf has a single critical timestamping EKU but never look at its basic constraints, so a certificate that asserts the CA bit while carrying that EKU is accepted as a TSA signer. crypto/x509 chain verification does not reject a CA certificate used as the leaf, so nothing else catches it, and that goes against RFC 3161 2.3 which expects the TSA certificate to be an end-entity and keeps certificate-issuing keys separate from timestamp-signing keys. This rejects a leaf whose basic constraints mark it as a CA in both the response-verification and issuance-chain paths.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.39%. Comparing base (6fd19b0) to head (4832d39).
⚠️ Report is 669 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1416       +/-   ##
===========================================
- Coverage   52.85%   26.39%   -26.46%     
===========================================
  Files          20       55       +35     
  Lines        1209     3171     +1962     
===========================================
+ Hits          639      837      +198     
- Misses        509     2282     +1773     
+ Partials       61       52        -9     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.