feat(python): async WebSocket-backed AsyncSerialMonitor methods

[zackees]

Closes #65.

Completes the async Python API begun in PRs #73, #78, #84. Adds the WebSocket-backed async methods on AsyncSerialMonitor that were deferred in PR #73's scaffold.

What's in

• __aenter__ / __aexit__ — async context manager opens /ws/serial-monitor, sends attach handshake, stores split sink/source halves, cleans up (Detach + Close) on exit
• read_lines(timeout_secs) — async batch read; honors auto_reconnect (Preempted → continue; Reconnected → continue)
• write(data) — async write, waits for daemon write_ack
• write_json_rpc(request, timeout_secs) — async JSON-RPC send + poll for REMOTE: response, preserving PR Honor full JSON-RPC wait timeout #57's full-timeout guarantee (empty batch doesn't short-circuit deadline)

Send+Sync refactor

The sync SerialMonitor wraps its WS state in std::sync::Mutex, which can't be held across .await. For the async surface:

• Split WebSocketStream via .split() into WsSink + WsSource
• Each half stored in its own Arc<tokio::sync::Mutex<Option<_>>> — concurrent read/write futures don't serialize each other, and each future owns its own Arc clone into future_into_py

Sync SerialMonitor is untouched — purely additive.

Test plan

• uv run cargo test -p fbuild-python --lib (11 existing tests passing)
• uv run cargo clippy --workspace --all-targets -- -D warnings
• uv run cargo fmt --all -- --check

Deferred

Unit tests for the new async methods. Follow-up would need a more elaborate WebSocket mock than the raw tokio::net::TcpListener HTTP mock used for send_op_async in PR #84.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between d37995b and e905230.

📒 Files selected for processing (1)

• crates/fbuild-python/src/lib.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/65-async-websocket-methods

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