fix: Serialise JWK properly in cnf claim

[jsnctl]

• Fixes JWK serialisation bug by passing ecdsa.PublicKey directly

Previously this would report the JWK as a symmetric oct key (which is forbidden for the WITs proof-of-possession mechanism)

   "jwk": {
      "alg": "ES256",
      "k": "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEu5lTCtfYxCwavDz5sGbaUEWjAUjeKDJFt93ziBKI2DHskPn3Fjq7IZS-S9A-l1IPzqkladSW-wYkpX1L6a9kvg",
      "kty": "oct",
      "use": "sig"
    }

This instead serialises the key info properly, and will return the WIT with the cnf more like:

   "jwk": {
      "alg": "ES256",
      "crv": "P-256",
      "kty": "EC",
      "use": "sig",
      "x": "-G08BQIgztYmr_N6-EZ7eezLqg5RieQGNSPExH9QMXg",
      "y": "ks5P9bPy7EPDru-znmk-jmD9iVRqCuSYWF2xhl9BId0"
    }

[gemini-code-assist]

Summary of Changes

Hello @jsnctl, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the serialization of JSON Web Keys (JWK) within the cnf claim for Workload Identity Tokens (WITs). Previously, the public key was incorrectly serialized as a symmetric key, violating the requirements for proof-of-possession. The changes ensure that the ecdsa.PublicKey is properly handled and serialized by directly passing the ecdsa.PublicKey object, resulting in a compliant JWK structure with appropriate elliptic curve parameters.

Highlights

• JWK Serialization Fix: Corrected an issue where JWK was incorrectly serialized as a symmetric "oct" key, which is forbidden for WITs proof-of-possession. The fix ensures proper serialization of elliptic curve public keys.
• generateWorkloadKeyPair Function Signature Update: Modified the generateWorkloadKeyPair function to directly return an *ecdsa.PublicKey object instead of its marshaled byte representation, facilitating correct JWK construction.
• Variable Renaming for Clarity: Renamed the privateKey variable to privateKeyBytes in MintWITSVID and generateWorkloadKeyPair for improved clarity, reflecting its byte slice type.

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[gemini-code-assist]

Code Review

This pull request addresses a bug in the serialization of the JSON Web Key (JWK) for the cnf claim in a WIT-SVID. The change correctly modifies generateWorkloadKeyPair to return a *ecdsa.PublicKey instead of a marshaled byte slice. This allows the go-jose library to properly serialize the key as an Elliptic Curve key, fixing the issue where it was previously identified as a symmetric oct key. The code changes are correct, and the renaming of variables like privateKey to privateKeyBytes improves readability. Overall, this is a good fix that effectively resolves the described problem.