Stabilize window frame compatibility test

[AArnott]

Summary

• replace the fixed 200 ms delay with synchronization on the relay observing a dropped window frame
• ensure the receiver starts draining only after the sender has actually exhausted its window

Root cause

TransferSucceeds_WhenWindowFramesAreDropped assumed its background writer would exhaust the channel window within 200 ms. Under load in the Windows .NET Framework test run, the writer could start late; once the receiver began reading, it kept up on the zero-latency pipe and no growth request was emitted. The final non-vacuous assertion then failed.

This is unrelated to #1148, which only changes TypeScript ESLint dependencies. The same test and assertion also failed on main in workflow run https://github.com/dotnet/Nerdbank.Streams/actions/runs/30677694206.

Validation

• affected test passes on net8.0 and net472
• affected test passes 20 consecutive net472 runs