pkcs12: PBKDF2-params prf is DEFAULT algid-hmacWithSHA1 - #2469

RFC 8018 (quoted in the type's doc comment) defines

prf AlgorithmIdentifier {{PBKDF2-PRFs}} DEFAULT algid-hmacWithSHA1

and DER omits a DEFAULT value, so PBKDF2 with HMAC-SHA1 is encoded without prf. openssl pkcs8 -v2prf hmacWithSHA1 produces this, as do older PKCS#12 tools. pkcs12::pbe_params::Pbkdf2Params declares prf as a required field, so these params fail to decode:

// PBKDF2-params from `openssl pkcs8 -topk8 -v2 aes-256-cbc -v2prf hmacWithSHA1`
let der = hex!("3016041027f144970f23ba4f9807dc9db52f00f102020800");
pkcs12::pbe_params::Pbkdf2Params::from_der(&der) // Err: ASN.1 DER message is incomplete
pkcs5::pbes2::Pbkdf2Params::from_der(&der)       // Ok, prf = HmacWithSha1

The fix decodes an absent prf as algid-hmacWithSHA1 ({ id-hmacWithSHA1, NULL }, RFC 8018 B.1.1) and omits it when encoding that value. The derive's default attribute only works for Copy fields (AlgorithmIdentifierOwned isn't), so DecodeValue/EncodeValue are written out, as pkcs5::pbes2::Pbkdf2Params already does. The public struct is unchanged.

Test: tests/pbe_params.rs decodes the OpenSSL params above, checks prf is HMAC-SHA1, and re-encodes byte for byte. It fails on master. The pkcs12.yml matrix passed locally: powerset tests on stable and 1.85, thumbv7em powerset build, fmt, clippy.

This PR was produced by AI agents (Claude) and reviewed by me before submission.