[OpenID4VCIFrontend] Security Vulnerability - Private Key Exposure in JWKS

In OpenID4VCIFrontend the JWKS (JSON Web Key Set) provided in Entity Configuration (`/.well-known/openid-federation`), includes private key parameters (e.g. d).
According to the [OpenID4VCI specification](https://openid.net/specs/openid-4-verifiable-credential-issuance-1_0.html), the JWKS must only expose public keys. These keys should never include sensitive private material.

**Impact:**
Exposing the private key completely breaks the trust model of the identity ecosystem.

**Metadata example:**
```
{
"iss": "https://eaa-provider.example.org",
"sub": "https://eaa-provider.example.org",
"jwks": {
"keys": [
{
"kty": "EC",
"d": "", // <--- ERROR: private element
"use": "",
"crv": "P-256",
"kid": "",
"x": "",
"y": "",
"alg": "ES256"
}
]
}
}
```