[Rust] Add multiplexed stream builder

[danilonajkov-db]

Summary

Builds on the lifecycle hardening merged in #826.

• Add StreamBuilder::multiplexed(n) (Beta) for JSON, compiled protobuf, and dynamic protobuf, available with default features.
• Open 1–64 homogeneous lanes concurrently with stable indices and bounded startup jitter (none for one lane). Divide the configured in-flight budget across lanes.
• Acquire each lane's gRPC client through the SDK helper, so connection_per_stream(true) creates dedicated transports and false retains shared-channel behavior.
• Preserve ordinary ack_callback(...) and provide a separate multiplexed_ack_callback(...) using MessageId. Validate callback/format mismatches before opening connections.
• Preserve lane-specific waits, all-lane flush, and close semantics from [Rust] Harden multiplexed stream lifecycle #826. Keep the existing testing-feature MultiplexedStream::new constructor.
• Guard cancelled construction and abort owned sender/receiver tasks when their supervisor is dropped. Ordinary Avro and Arrow telemetry support remains intact; mux formats are limited to JSON/protobuf.
• Document usage, ordering, capacity, lifecycle, and callback behavior, with a complete compiled-protobuf example.

Use multiplexing when one gRPC stream limits throughput and global ordering is unnecessary.

Validation

• make test — workspace tests, all-feature/default-feature SDK tests, doctests, and all-feature JNI tests
• make lint — full repository Clippy matrix with warnings denied
• cargo check --offline --locked -p rust-examples-proto --example proto_compiled_multiplexed
• cargo fmt --all --check and git diff --check
• Locked dependency resolution for Python and TypeScript; the builder reuses the repository's existing rand 0.9 resolution

Focused regressions cover callback-mode validation, lane-count/capacity validation, Avro rejection, cancelled construction, and dedicated versus shared mux connections.