Add .NET system Nexus serialization context

[tconley1428]

Summary

• Add the .NET serialization-context factory annotation for the Signal-with-Start System Nexus operation.
• Add .NET from-proto conversion metadata for native Temporal model types.
• Represent the two workflow-ID policy fields as proto-backed aliases and add the native failure type.

These annotations let nexgen generate the operation metadata and native .NET conversions needed by the SDK.

Validation

• git diff --check
• cargo build-examples --lang dotnet (nexgen example regeneration)