zebrad vulnerable to full node denial of service via crafted Sapling receiver in z_listunifiedreceivers

### Am I affected You are affected if: 1. You run `zebrad` up to and including `v4.4.1`. 2. Your `zebrad.toml` sets `rpc.listen_addr` to a TCP address (RPC server is enabled). 3. An attacker can authenticate to the RPC endpoint. With the default `enable_cookie_auth = true`, this requires the attacker to read the `.cookie` file (typically local access). With `enable_cookie_auth = false`, any network client reaching the RPC port can trigger it. ### Summary The `z_listunifiedreceivers` RPC handler panics when processing a structurally valid Unified Address whose Sapling receiver carries 43 bytes that fail cryptographic validation (`sapling_crypto::PaymentAddress::from_bytes` returns `None` for non-subgroup Jubjub points). The handler calls `.expect("using data already decoded as valid")` on the fallible result. Because Zebra's release profile sets `panic = "abort"`, the panic terminates the entire node process, not just the RPC task. ### Details `zcash_address::unified::Encoding::decode` validates only the structural envelope of a Unified Address (F4Jumble, bech32m, typecode ordering, 43-byte length for Sapling). It does not validate that the embedded `pk_d` is a valid Jubjub subgroup point or that the diversifier produces a valid `g_d` preimage. At `zebra-rpc/src/methods.rs:2893`, the handler calls `Address::try_from_sapling(network, data)`, which delegates to `sapling_crypto::PaymentAddress::from_bytes`. When `from_bytes` returns `None` (most random 32-byte strings fail the su