Require server.version before any other Electrum RPC method

[PeterXMR]

Refs #1241 — checks off the server.version() must-be-first-message item on the v1.6 protocol checklist.

Summary

Electrum protocol v1.6 mandates that server.version is the first message a client sends. Enforce this in electrs.

• Adds negotiated_version: Option<String> to the per-client state (Client)
• Set inside fn version() once the version handshake succeeds
• In single_call, reject any non-Version call when negotiated_version.is_none() with a JSON-RPC error that names the required method

⚠️ Behavior change

Any client that today sends server.ping / server.banner / server.features / etc. before the version handshake will start receiving an error. This is a deliberate v1.6 spec-compliance change.

Happy to soften this to a warn!-then-allow if you'd prefer a phased rollout — let me know.

Test plan

• New unit test test_client_starts_without_negotiated_version confirms the default-None state the gate keys on
• cargo test --lib — 22/22 pass
• cargo clippy --all-targets -- -D warnings — clean
• cargo fmt --check — clean
• Live tested against Bitcoin Core v31.0.0 on regtest, three scenarios:

$ # A. pre-version calls should be rejected
$ printf '{"id":1,"method":"server.ping","params":[]}
{"id":2,"method":"server.banner","params":[]}
{"id":3,"method":"server.features","params":[]}
' | nc -w 2 127.0.0.1 60401
{"error":{"code":1,"message":"`server.version` must be called before any other method"},"id":1,"jsonrpc":"2.0"}
{"error":{"code":1,"message":"`server.version` must be called before any other method"},"id":2,"jsonrpc":"2.0"}
{"error":{"code":1,"message":"`server.version` must be called before any other method"},"id":3,"jsonrpc":"2.0"}

$ # B. version first, then a regular call should succeed
$ printf '{"id":4,"method":"server.version","params":["test","1.4"]}
{"id":5,"method":"server.ping","params":[]}
{"id":6,"method":"server.banner","params":[]}
' | nc -w 2 127.0.0.1 60401
{"id":4,"jsonrpc":"2.0","result":["electrs/0.11.1","1.4"]}
{"id":5,"jsonrpc":"2.0","result":null}
{"id":6,"jsonrpc":"2.0","result":"Welcome to electrs 0.11.1 (Electrum Rust Server)!"}

$ # C. batch with version first also works (state propagates within the batch)
$ printf '[{"id":7,"method":"server.version","params":["test","1.4"]},{"id":8,"method":"server.ping","params":[]}]
' | nc -w 2 127.0.0.1 60401
[{"id":7,"jsonrpc":"2.0","result":["electrs/0.11.1","1.4"]},{"id":8,"jsonrpc":"2.0","result":null}]