test+fix: recover MockGateway-deletion coverage; fix async cancel deadlock

[wboayue]

Summary

After PR #480 deleted the MockGateway-driven test files, coverage dropped in a handful of files. This PR recovers it via two phases:

1. Targeted unit tests (MessageBusStub-driven) for trivial Client accessors and MessageBus-trait impl methods on the real TcpMessageBus / AsyncTcpMessageBus.
2. Minimal handshake-replay listeners (spawn_handshake_listener per side, ~50 LOC each) that bind to 127.0.0.1:0 and let Client::connect* exercise the production TCP paths without restoring MockGateway's per-API-call surface.

Also fixes a real production deadlock found while writing the trait-level cancel tests.

Coverage delta

cargo llvm-cov --all-features --summary-only:

File	Region %	Line %
TOTAL	82.65% → 83.80% (+1.15)	87.21% → 88.21% (+1.00)
client/sync.rs	52.74 → 90.41 (+37.67)	61.54 → 93.27 (+31.73)
client/async.rs	62.59 → 95.92 (+33.33)	73.45 → 97.35 (+23.90)
transport/async_io.rs	0.00 → 57.89 (+57.89)	0.00 → 71.05 (+71.05)
connection/async.rs	66.67 → 73.64 (+6.97)	72.14 → 77.86 (+5.72)
connection/sync.rs	84.08 → 88.57 (+4.49)	92.14 → 95.00 (+2.86)
transport/sync/mod.rs	59.02 → 69.81 (+10.79)	62.06 → 72.52 (+10.46)
transport/async.rs	44.26 → 51.83 (+7.57)	46.14 → 53.88 (+7.74)

What's added

Phase 1 — MessageBusStub-driven unit tests

• src/client/{sync,async}_tests.rs — sibling test files exercising Client::stubbed-built clients: accessor round-trip, check_server_version branches, builder factories, send_* helpers, create_order_update_subscription uniqueness.
• transport/sync/tests.rs — 5 tests calling MessageBus-trait methods on the real TcpMessageBus<MemoryStream>: cancel_subscription, cancel_order_subscription, cancel_shared_subscription, send_message, create_order_update_subscription.
• transport/async_tests.rs — 5 mirroring tests on the real AsyncTcpMessageBus<MemoryStream> plus an is_connected reflection check.

Phase 2 — Handshake-replay TCP listener

• src/transport/sync/test_listener.rs + src/transport/async_test_listener.rs — spawn_handshake_listener(frames) -> (SocketAddr, JoinHandle). Binds 127.0.0.1:0, accepts once, consumes the API\0 magic + version range + start_api writes, replays length-prefixed frames, then drains further writes until the client closes.
• 3 sync + 3 async tests at client/{sync,async}_tests.rs:
  • connect_handshakes_against_real_socket — Client::connect succeeds, metadata populated.
  • connect_with_callback_receives_unsolicited_messages — injects an OpenOrder mid-handshake; asserts the callback fires.
  • connect_with_options_applies_tcp_no_delay — connect_with_options(tcp_no_delay=true) returns Ok.

What's fixed

AsyncTcpMessageBus::cancel_subscription and cancel_order_subscription self-deadlocked: each acquired a read guard, then awaited a write guard on the same RwLock while the read guard was still alive (Rust shadowing doesn't drop the prior binding before the new RHS evaluates).

// before
let channels = self.request_channels.read().await;          // read guard alive
if let Some(sender) = channels.get(...) { ... }
let mut channels = self.request_channels.write().await;     // deadlock here

The bug was latent because the production cancel flow goes through Subscription::cancel().await → message_bus.send_message(...), not through cancel_subscription. The new test_cancel_subscription_writes_and_clears_channel was the first caller to hit the path on the real bus and hung. Fix collapses to a single write guard.

Backport tracked in #483 (v2-stable has the identical bug).

Remaining gaps (intentionally accepted)

• transport/async.rs 51.83% — process_messages task body branches (timeout-loop, connection-error reconnect) need more sophisticated test harnesses; deferred.
• transport/async_io.rs 57.89% — AsyncTcpSocket::reconnect and sleep (the listener accepts once, can't drive reconnect cleanly without listener swap).
• transport/routing.rs 72.33% — routing decision branches; orthogonal to MockGateway deletion.

These are covered by the live-Gateway integration tests under tests/.

Test plan

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo clippy --all-targets --features sync -- -D warnings
• cargo clippy --all-features
• cargo test (default): 877 passed (+13 from main)
• cargo test --no-default-features --features sync: 881 passed (+13)
• cargo test --all-features: 1058 passed (+26)