APATs have non-standard signature

[hannes-ucsc]

Created by Claude Code

The signature verification failure on jwt.io was caused by a bug in KMSSigningAlgorithm: KMS returns DER-encoded ECDSA signatures, but RFC 7518 Section 3.4 requires JWS to use raw r||s format. The APAT signature was valid but non-standard, so external JWT libraries (including jwt.io) rejected it.

Originally posted by @hannes-ucsc in #8007

This issue is does not weaken the security of the APAT mechanism but eventually we should still use a standard signature format.

[hannes-ucsc]

For demo, obtain an APAT, revoke the APAT and show that jwt.io considers the signature valid.