[Rust] Test Arrow mux lifecycle

[danilonajkov-db]

What changes are proposed in this pull request?

Add nine Arrow mux integration tests covering terminal poisoning, healthy-lane waits, partial-ack suffix retrieval, retryable replay, mux-wide backpressure, capacity timeout and cancellation, interrupted close/recovery, construction cleanup, and poisoned capacity waiters. The mock Flight server can script individual connections so failures and healthy sibling lanes can be exercised independently. Timing-sensitive cases use virtual time and synchronization barriers.

This PR changes tests only. Stack base: #866.

How is this tested?

• Complete Arrow integration suite: 113 passed, including all 11 Arrow mux tests and two standalone Arrow admission regressions.
• Existing gRPC mux integration suite: 24 passed.
• Full Rust make test and make lint matrices passed for the combined stack.
• Standalone Avro multiplexing regression test passed.
• Rust formatting and diff whitespace checks passed.
• The final stack retains the same runtime code and tests validated by the full matrix above.