Function decrypt_block_to_plaintext 

pub fn decrypt_block_to_plaintext(
    store: &dyn BlockStore,
    crypto: &dyn CryptoEngine,
    codec: &PostcardCodec,
    block_id: u64,
) -> FsResult<Vec<u8>>

Read a block, extract envelope, decrypt, and return the raw plaintext bytes (before deserialization into a typed object). Used by the object cache.