fix(rpc): stop get_output_distribution JSON-RPC panic

[orrinfrazier]

Closes #11

Summary

• The get_output_distribution JSON-RPC handler panicked on every call via a todo!(), exposing a remote DoS on any node with RPC enabled (the default restricted server).
• Routes the dispatch arm to not_available(), matching the /get_output_distribution.bin variant, until the binary-string distribution encoding is implemented.

Why

json_rpc.rs dispatched GetOutputDistribution to shared::get_output_distribution, which does real DB work and then builds its response with distributions: todo!(\"...binary strings: monero-project/monero#9422\"). The todo!() unconditionally panics the RPC task. The only guard rejects non-RCT amounts, so a restricted/remote client requesting RCT outputs (amounts == [0]) passes it and reaches the panic.

Changes

• binaries/cuprated/src/rpc/handlers/json_rpc.rs: GetOutputDistribution now routes to not_available()? (one match arm), mirroring bin.rs:53. The now-unreachable json_rpc::get_output_distribution wrapper and shared::get_output_distribution are left in place (matching the existing bin::get_output_distribution precedent; covered by the crate-wide #![allow(dead_code)]), preserving the partial impl for when [Proposal] Deprecate RPC binary strings monero-project/monero#9422 lands.

Testing

• `cargo clippy -p cuprated --all-targets` — clean, exit 0 (workspace denies clippy warnings).
• `cargo fmt -p cuprated --check` — clean.
• No unit test added: `CupratedRpcHandler` cannot be constructed without launching a full node (its service handles have no public/test constructor) and the repo has no RPC test harness. The fix is verified by symmetry with the already-shipping `bin.rs:53` pattern and the ~20 sibling `not_available()?` arms in the same match.

Fixes the panic described in `issues/S02-1-get-output-distribution-panic.md` (survey TOP-5 #3).