fix(dotnet): recover from dropped session idle events

[hydraxman]

Summary

• make .NET SendAndWaitAsync recover when the ephemeral session.idle notification is dropped
• use the runtime's task-drain and activity RPCs as the authoritative fallback
• flush the SDK's FIFO event-dispatch queue and reject stale inactive snapshots when new events or work appear during confirmation

Reproduction

• Before: the dropped-idle unit reproducer times out after receiving the assistant response and assistant.turn_end
• After: the same reproducer completes after the runtime reports no active work and preceding user handlers have drained
• A second deterministic reproducer reactivates runtime work during the final dispatch barrier and verifies the stale inactive snapshot is not used

Tests

• dotnet test test/GitHub.Copilot.SDK.Test.csproj -f net8.0 --no-restore --filter 'FullyQualifiedName~SendAndWaitAsync_Completes_When_SessionIdle_Notification_Is_Dropped|FullyQualifiedName~SendAndWaitAsync_DroppedIdle_Fallback_Flushes_Preceding_Event_Handlers|FullyQualifiedName~SendAndWaitAsync_DroppedIdle_Fallback_Rechecks_Activity_After_Final_Barrier|FullyQualifiedName~SendAndWaitReliabilityE2ETests'
• dotnet test test/GitHub.Copilot.SDK.Test.csproj -f net8.0 --no-restore — 676 passed, 3 skipped
• COPILOT_SDK_DEFAULT_CONNECTION=inprocess dotnet test test/GitHub.Copilot.SDK.Test.csproj -f net8.0 --no-restore --filter 'FullyQualifiedName~SendAndWaitReliabilityE2ETests'
• dotnet build src/GitHub.Copilot.SDK.csproj --no-restore — 0 warnings, 0 errors
• dotnet format --verify-no-changes --no-restore

Fixes #2008