Make queued client operations cancellation-aware and bounded

[ryanfowler]

Summary

• Skip queued async commands when their reply oneshot has already been canceled, so dropped or timed-out futures do not run later.
• Add ClientBuilder::queue_capacity() and PoolBuilder::queue_capacity() to bound worker queues.
• Return Error::QueueFull when a bounded queue is saturated, and switch enqueue paths to nonblocking try_send.
• Add regression coverage for canceled queued work and full-queue behavior.

Testing

• cargo fmt --all -- --check
• cargo build
• cargo test
• cargo clippy -- -D warnings