Add opt-in unversioned fallback for versioned task dispatch

[torosent]

Summary

• Add opt-in unversioned fallback for versioned orchestration and activity dispatch when no exact version match exists.
• Keep existing closed-set behavior by default, while wiring fallback-aware factory dispatch, UseWorkItemFilters(), and startup warning guidance.
• Add UnversionedFallbackSample demonstrating exact-version dispatch and unmatched-version fallback with the DTS emulator.

Compatibility

No breaking change — the new behavior is opt-in and the default remains the existing closed-set version dispatch behavior. Strict worker versioning remains a pre-dispatch gate and is not bypassed by fallback.

Validation

• dotnet build samples/UnversionedFallbackSample/UnversionedFallbackSample.csproj --configuration Debug --verbosity minimal
• DTS emulator sample run with DURABLE_TASK_SCHEDULER_CONNECTION_STRING='Endpoint=http://localhost:8090;TaskHub=default;Authentication=None'
• dotnet test test/Worker/Core.Tests/Worker.Tests.csproj --configuration Debug --verbosity minimal
• dotnet test test/Worker/Grpc.Tests/Worker.Grpc.Tests.csproj --configuration Debug --verbosity minimal
• git diff --check