fix(graph): resolve startup deadlock on liveliness query with many live tokens

[YuanYuYuan]

Summary

• New nodes joining a running system with many already-live nodes/tokens could deadlock on startup inside ZContextBuilder::build(). Fixes this by giving Graph::new_with_pattern's liveliness query a large enough FIFO capacity that the deadlock condition can no longer occur.

Root cause

Graph::new_with_pattern (crates/hiroz/src/graph.rs) issues a blocking liveliness query during context startup to discover the existing ROS graph from other peers' liveliness tokens:

let replies = session.liveliness().get(&pattern).timeout(...).wait()?;

Zenoh's Session::liveliness_query synchronously replays every currently-live token matching the pattern as a reply, on the calling thread, before the call returns. The default handler backing .wait() is a bounded flume channel with capacity 256. Because the replay happens before liveliness_query() returns, the caller's own drain loop can't start running until the sender-side burst is already complete — if more than 256 live tokens match the pattern, the synchronous send blocks forever waiting for a receiver that can't run yet. Deterministic self-deadlock once live-token count exceeds capacity, matching the reported ~30% hang rate (live-token count hovering near the 256 threshold across runs).

This is a zenoh-core design issue rather than something specific to hiroz's usage; a fix is being pursued upstream at the zenoh level as well.

Fix

Graph::new_with_pattern now passes an explicit, oversized FifoChannel handler to the liveliness query instead of relying on zenoh's small default:

let replies = session
    .liveliness()
    .get(&pattern)
    .with(zenoh::handlers::FifoChannel::new(65536))
    .timeout(...)
    .wait()?;

65536 is a generous ceiling that eliminates the deadlock class at any node/token count realistically seen in a ROS graph.

Test plan

• cargo test -p hiroz — full suite green, including graph/action discovery tests exercising the liveliness query path.

Breaking Changes

None.