fix(p256): prevent secret serialization in DID document

[eqty-devbot]

Summary

• set serialize_secrets to false in P256Signer::new and P256Signer::import
• ensure generated DID documents do not include private key material
• keep JOSE formatting behavior unchanged

Why

We should never serialize secrets into DID documents. This change explicitly disables secret serialization for P-256 signer DID document generation.

Notes

• This supersedes any prior intent to serialize secrets in this code path.

[cameronfyfe]

😞