Investigate Copilot SDK cancellation serialization bug #319

Summary

Multi-agent orchestration sessions get stuck at IsProcessing=true when cancellation tokens fire. The root cause is a bug in the Copilot SDK where StreamJsonRpc.RequestId is not included in the AOT-safe JSON serialization context.

Symptoms

• Orchestrator sessions show Thinking... forever
• Crash log shows NotSupportedException for RequestId serialization at StreamJsonRpc.StandardCancellationStrategy.CancelOutboundRequest
• 180s watchdog fires, response length is 0

Root Cause

SDK Bug: When a CancellationToken fires during an SDK call, StandardCancellationStrategy tries to serialize RequestId, but the SDK's SystemTextJsonFormatter doesn't include it in the TypeInfoResolverChain. The exception is unobserved, killing the JSON-RPC reader silently.

Workaround

PR implements workaround: pass CancellationToken.None to SDK SendAsync calls. Cancellation is handled at the TCS level via ResponseCompletion.TrySetCanceled().

Action Items

• Decide whether to file issue on github/copilot-sdk
• Monitor for SDK fix
• Remove workaround once fixed

SDK Versions Affected

• Broken: 0.1.30, 0.1.32
• Works: 0.1.26