Produce fixed-sized EC JWKs in tests

[maxtropets]

Failure in https://github.com/microsoft/CCF/actions/runs/27386294910/job/80933917383

In #7924, a constitution hardening took place, after which truncated keys are no longer accepted, but we sometimes emit them in tests.

This PR makes key encoding deterministic, as well as adds a quick test-check.

[Copilot]

Pull request overview

This PR updates the Python JWT issuer test utilities to emit ES256 JWK x/y coordinates as fixed-width (32-byte) base64url values, and adds a regression assertion to ensure leading-zero coordinates are preserved. This aligns the test-generated JWKS with the stricter constitution validation introduced in #7924 (which rejects truncated coordinates).

Changes:

• Pad ES256 JWK x/y coordinates to a fixed 32-byte width before base64url encoding.
• Add a targeted test assertion using fixed EC public numbers to verify leading-zero preservation and fixed-width output.

Custom instructions used:

• .github/copilot-instructions.md
• .github/instructions/reviewing.instructions.md

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/js-custom-authorization/custom_authorization.py	Adds a regression assertion ensuring ES256 raw-key JWKS coordinates are emitted at fixed width (including leading-zero cases).
tests/infra/jwt_issuer.py	Updates JWK numeric encoding helper usage so ES256 x/y are encoded as fixed 32-byte values.