fix: bound BasicConstraints ASN.1 parsing

[leanthebean]

Summary

Fixes CAT finding 7596fd8c-ca58-496f-81bc-c8e71c2ad57a.

The BasicConstraints verifier previously called firstChildOf on the decoded extension value unconditionally. For an empty BasicConstraints SEQUENCE (30 00), that made the ASN.1 walker parse the next byte outside the BasicConstraints container, and the escaped node could influence the cA / pathLenConstraint decision.

This change:

• treats an empty BasicConstraints SEQUENCE as cA=false without traversing children;
• bounds every parsed BasicConstraints child against the extension SEQUENCE end before consuming it;
• rejects unknown BasicConstraints fields and trailing fields after pathLenConstraint;
• keeps valid encodings supported: client 30 00, CA 30 03 01 01 ff, and CA with pathLen 30 06 01 01 ff 02 01 00.

Security Impact

This removes the parser-bounds escape from the PKI trust gate. The existing coincidental mitigations still made the finding non-weaponizable against the current AWS Nitro profile, but the verifier no longer relies on adjacent extension ordering or outer signature-algorithm bytes for safety.

Tests

• forge test --match-path test/CertManager.t.sol -vvv
• forge test

[leopoldjoy]

Reject zero-length pathLenConstraint before calling uintAt; otherwise an in-bounds 02 00 child still makes uintAt read ptr.content() outside the BasicConstraints SEQUENCE and can accept the malformed INTEGER as 0.