Function verify_p256_signature_internal 

fn verify_p256_signature_internal(
    r: &[u8],
    s: &[u8],
    pub_key_x: &[u8],
    pub_key_y: &[u8],
    message_hash: &B256,
) -> Result<(), &'static str>

Verifies a P256 signature using the provided components

This performs actual cryptographic verification of the P256 signature according to the spec. Called during recover_signer() to ensure only valid signatures enter the mempool.