New checkpoint request protocol

[LucDeCaf]

Implements the new Checkpoint Requests protocol for .NET, using the other SDKs as reference (mostly JS and Kotlin).

This PR also includes a internal refactor to the StreamingSyncImplementation class, which shifts it closer to the Kotlin SDK's implementation rather than JavaScript's.

Justification: While C# syntax looks a lot like JavaScript syntax, C#/.NET acts a lot more like Kotlin/Java internally. The new implementation takes better advantage of C#'s Task model and is generally cleaner/easier to reason about.

Note that this uses strings for the checkpoint request ID despite C# having a fast, native 64-bit integer type. This is mostly for consistency with other internal APIs and to avoid any accidental public API changes, but I'd like to use longs for IDs in v1.

AI Usage

I used Claude Code to assist with the StreamingSyncImplementation refactor and with porting tests/test utils from Kotlin/JS to C#. I reviewed the changes manually.

[simolus3]

I wanted to leave a few early comments in case it helps. As far as the public API is concerned this looks good to me.

[simolus3]

It looks like this might not be thread-safe. It's annoying that there is no MutableStateFlow-equivalent, but maybe the Mutex-based implementation from Swift could be easier to port?

[simolus3]

The main reason for this layer of indication in JavaScript was that we otherwise can't cancel waiting for status updates.

ChannelReader does not have that issue, is it possible to structure this method like (pseudo-C#):

var reader = _stateBroadcaster.Subscribe(out var subscriberId);

try {
  if (HandleState(_state)) return;

  while (!HandleState(reader.WaitToReadAsync(signal))) {}
} finally {
  // ... also remove this from HandleState
  _stateBroadcaster.Unsubscribe(subscriberId);
}

[LucDeCaf]

For some reason I thought WaitToReadAsync(CancellationToken) wasn't available on netstandard2.0 (it is). Probably getting it confused with another cancellable Stream method.