fix(clob): keep heartbeats alive across client clones

[mattfaltyn]

Summary

• cancel the automatic heartbeat task only when the final DroppingCancellationToken owner is dropped
• keep heartbeats running when temporary Client clones, including order-builder clones, are consumed or dropped
• add a bounded-polling integration regression that verifies heartbeats continue after dropping a limit-order builder and can still be stopped explicitly

Root cause

Client and DroppingCancellationToken both derive Clone, but every cloned wrapper previously called CancellationToken::cancel() from Drop. OrderBuilder embeds a cloned client, so completing or dropping a normal order builder silently terminated the shared heartbeat task while the original client continued reporting heartbeats_active() == true.

The fix uses Arc::into_inner to atomically identify the final receiver owner. This preserves existing explicit-stop and multiple-owner synchronization behavior while ensuring exactly one final drop performs cancellation, including under concurrent drops.

Impact

Heartbeat-enabled clients can build orders without silently stopping the dead-man heartbeat mechanism and risking order-priority loss or cancellation of open orders.

Validation

• cargo build --all-targets --all-features
• cargo test — 15 unit tests and 4 doctests passed; feature-gated integrations correctly skipped
• cargo +nightly-2025-11-24 fmt --all -- --check
• cargo +1.88 clippy --all-targets --all-features -- -D warnings
• cargo +1.88 clippy --all-targets -- -D warnings
• cargo sort --check with cargo-sort 2.0.2
• cargo +1.88 llvm-cov --all-features --workspace --lcov --output-path lcov.info — 559 all-feature tests passed
• PRE_COMMIT_HOME=/tmp/pre-commit pre-commit run --files src/clob/client.rs tests/clob.rs
• focused heartbeat regression passed 20 consecutive runs

Fixes #91

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Note

Medium Risk
Touches shared heartbeat teardown for all authenticated client clones; wrong refcount logic could leave heartbeats running or stop them too early, but scope is narrow and covered by a new regression test.

Overview
Fixes a bug where dropping any cloned Client (including a temporary limit-order builder) cancelled the shared heartbeat task while the main client still reported heartbeats as active.

DroppingCancellationToken’s Drop now calls CancellationToken::cancel() only when the dropped instance holds the last Arc to the cleanup receiver (Arc::into_inner), so intermediate clones from limit_order() / market_order() no longer stop the background heartbeat loop. Explicit stop_heartbeats / cancel_and_wait behavior is unchanged.

Adds a heartbeats integration test that drops a limit-order builder, asserts heartbeats keep posting, then verifies stop_heartbeats still shuts them down.

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