[Rust] Add Arrow mux API

[danilonajkov-db]

What changes are proposed in this pull request?

Add Rust-only multiplexed Arrow Flight ingestion through .arrow(schema).multiplexed(n).build_arrow(), returning MultiplexedArrowStream. Whole RecordBatches route round-robin, MessageId identifies the lane and local batch offset, and the mux-wide max_inflight_batches budget is divided evenly among lanes. No global ordering is promised.

Private Arrow reservation and admission hooks connect to the shared core from #865, inheriting its poisoning and cancellation handling. Close preserves an already-recorded mux failure; otherwise it returns the first observed lane-close error and logs additional errors. Arrow retains its recovery and close supervisor. All lanes share the stats exporter; events retain lane-local offsets. Arrow acknowledgment callbacks remain unsupported.

For standalone Arrow streams, ingest_batch calls blocked on max_inflight_batches now fail as soon as close() starts instead of waiting for close to finish. Other closed-admission checks retain the prompt rejection behavior already present in 2.10. Includes focused tests for both cases, builder validation, routing/IPC/message-ID/telemetry tests, API docs, a runnable example, and changelog entries. No FFI or wrapper API changes.

Stack base: #865. Follow-up: #867 adds broader lifecycle and failure tests.

How is this tested?

• Arrow integration suite at this layer: 104 passed.
• SDK unit tests with all features: 388 passed, 1 ignored.
• Full Rust make test and make lint matrices passed on the combined implementation, plus the standalone Avro mux regression.
• Rust formatting and diff whitespace checks passed.