Add .NET Workflow Streams samples

[brianstrauch]

Summary

• add six Workflow Streams scenarios covering workflow/activity publishing, listener callbacks with backpressure, reconnecting subscribers, external publishers, truncation, and LLM token streaming
• add focused integration tests and repository documentation
• add SDK and AI SDK ownership for the sample and tests
• target the Workflow Streams API introduced by Add Workflow Streams extension library sdk-dotnet#804

Validation

• dotnet build tests/TemporalioSamples.Tests.csproj: 0 warnings, 0 errors
• focused Workflow Streams tests: 6 passed
• dotnet format --verify-no-changes
• git diff --check

Dependency note

This targets Temporalio 1.19.0 and Temporalio.Extensions.WorkflowStreams 1.19.0. Validation used packages built locally from the SDK PR because those versions are not yet published.