Use client-generated request_async_id for Unity FFI async requests and replace delegate fan-out with pending callback dispatch

[xianshijing-lk]

Description
This PR updates the Unity FFI async request path to use client-generated request_async_id end-to-end instead of relying on Rust-generated async_id for request/callback matching.

What changed

• Generate request_async_id on the Unity side before sending async-capable FFI requests.
• Register async completions before crossing the FFI boundary to remove the callback-listener race.
• Replace per-request event subscription/fan-out with a single pending-callback map keyed by request_async_id.
• Dispatch async completions through one generic lookup path in FfiClient.
• Add cancellation/cleanup for pending async requests on send failure and client disposal.
• Keep unsolicited/event-stream callbacks (RoomEvent, TrackEvent, incremental stream events, RPC invocation events) on the existing event path.

Why
Previously Unity sent the request first, then relied on Rust’s returned async_id to subscribe for the async callback. If Rust completed quickly, the callback could arrive before Unity had registered its listener.

This PR removes that race by:

• generating the ID on the client,
• registering the pending completion first,
• then sending the request.

It also improves callback dispatch efficiency by avoiding multicast delegate scans when many async requests are in flight.

Behavioral impact

• Async request/response flows now match on request_async_id.
• Multiple concurrent async requests of the same type can complete independently and out of order.
• Pending one-shot completions are resolved or canceled at most once.
• Failed sends no longer leave hanging pending waiters.

Affected areas

• Room connect async completion
• Track stats async completion
• Local participant async operations:
  • publish/unpublish track
  • set metadata/name/attributes
  • perform RPC
  • send text/file
  • open text/byte streams
• Data stream read-all / write / close / write-to-file operations
• Audio frame capture async completion

Implementation notes

• FfiRequestExtensions.InitializeRequestAsyncId now assigns RequestAsyncId generically using cached runtime reflection rather than a large generated-type switch.
• FfiClient now owns a generic pending callback registry keyed by request_async_id.
• FfiRequestWrap.Send() cancels the pending callback if the send path throws before completion can arrive.

Testing

[MaxHeimbrock]

Works for me, though I don't have a full picture yet how the whole life cycle of requests and response and callbacks work. I tested on my Mac.

[MaxHeimbrock]

Approved.

[xianshijing-lk]

Thanks @MaxHeimbrock.

We will need to do more tests once your example gets landed, and we will make more tests / examples before making a new release