Verify FROST signatures before returning to caller

[kwsantiago]

After signature aggregation in try_aggregate(), verify the signature against the message and group pubkey before returning.

Risk: Without verification, invalid signatures could be returned due to malicious peers or bugs.

Scope: keep-frost-net/src/session.rs:202-233, keep-enclave/enclave/src/signer.rs

Add frost_secp256k1_tr::Signature::verify() call before returning aggregated signature.