fix(sdk): keep a shut down client shut down - #4374

Which issue does this PR address?

Relates to #4287

Rationale

shutdown() must be final, as agreed on #4287: a client that is shut down must not connect again, whatever the caller does next. Problems 1 and 4 of #4287 break this rule. This PR fixes these two problems. Problems 2 and 3, about disconnect(), come in a separate PR.

What changed?

A client that was shut down could come back. WebSocketClient dialed again on connect(), because its connect_inner had no Shutdown arm. On every transport, a disconnect() after shutdown() wrote Disconnected over Shutdown, so connect() worked again.

Now one guard in set_state keeps Shutdown final on TCP, QUIC and WebSocket, so no later write can revive the client. This guard also covers the Disconnected writes that other paths add, for example a timeout. WebSocketClient::connect_inner also returns ClientShutdown before any dial, as TCP and QUIC already do. The guard alone does not stop that dial.

The Client trait now documents this contract on shutdown(). The two lifecycle tests from #4344 for these cases lose their #[ignore]. On the Python side, the strict xfail of the WebSocket case and the known-issue note of shutdown() go away, because the bug is fixed.

Local Execution

I used Zed in devcontainer mode. The build and the tests ran in the devcontainer, and prek ran on macOS.

• Passed: cargo fmt --all and cargo clippy -p iggy_common -p iggy -p integration --all-features --all-targets -- -D warnings.

• Passed: cargo test -p iggy --lib, with 232 tests.

• Passed: the lifecycle tests. The 4 cases of problems 1 and 4 pass, and the 6 cases of problems 2 and 3 stay ignored:

  $ cargo test -p integration -- sdk::client_lifecycle
  test result: ok. 4 passed; 0 failed; 6 ignored; 0 measured; 991 filtered out
  

• Passed: the sdk:: integration suite (63 passed, 6 ignored), in two runs, and the cluster:: suite (80 passed), because the guard changes state transitions that reconnects and failovers use.

• Passed: the Python lifecycle tests against a local server, with no xfail left:

  $ uv run --no-sync pytest tests/test_connectivity.py -k Lifecycle
  16 passed, 35 deselected
  

• Passed: cargo run --bin stub_gen. The stub matches the edited docstring.

• Passed: pre-commit hooks with SKIP=web-lint prek run --all-files. I skipped web-lint because this PR does not change web/.

AI Usage

1. No