ssh-key: fix Signature::encode for sk-ecdsa-sha2-nistp256

[jkolo]

Problem

Signature's Decode and Encode impls are asymmetric for
sk-ecdsa-sha2-nistp256@openssh.com signatures.

Decode reads the security-key trailer (byte flags || uint32 counter)
for both SK algorithms:

if algorithm == Algorithm::SkEd25519 || algorithm == Algorithm::SkEcdsaSha2NistP256 {
    let flags = u8::decode(reader)?;
    let counter = u32::decode(reader)?;
    ...
}

But Encode only special-cases SkEd25519:

if self.algorithm == Algorithm::SkEd25519 {
    // split signature body from trailer, write them separately
} else {
    self.as_bytes().encode(writer)?;   // <-- sk-ecdsa falls through here
}

For an sk-ecdsa-sha2-nistp256@openssh.com signature this writes the
whole data buffer (ECDSA signature plus the 5-byte trailer) as a
single length-prefixed string, instead of the wire layout required by
OpenSSH's PROTOCOL.u2f:

string  ecdsa_signature
byte    flags
uint32  counter

A signature produced this way fails to decode again — the trailer bytes
end up buried inside the inner string — and is rejected by OpenSSH as an
invalid signature.

Fix

Extend the Encode sk-trailer branch to also match
SkEcdsaSha2NistP256. The existing trailer-splitting logic is generic
over SK_SIGNATURE_TRAILER_SIZE and works unchanged for both algorithms.

Tests

Adds decode_sk_ecdsa_sha2_p256 and encode_sk_ecdsa_sha2_p256,
mirroring the existing sk-ed25519 tests. The test vector reuses the
ECDSA signature components from ECDSA_SHA2_P256_SIGNATURE with an
sk-ecdsa algorithm prefix and a flags/counter trailer.
encode_sk_ecdsa_sha2_p256 fails before this change (encode emits a
72-byte body as a 77-byte string) and passes after.