Add test coverage for LargePayloads capability registration

[Copilot]

Addresses missing test coverage for capability registration when calling UseExternalizedPayloads(). The extension method adds WorkerCapability.LargePayloads to worker options but lacked verification.

Changes

• Added DurableTaskWorkerBuilderExtensionsAzureBlobPayloadsTests.cs with unit tests covering both UseExternalizedPayloads() overloads
• Tests verify WorkerCapability.LargePayloads is correctly added to GrpcDurableTaskWorkerOptions.Capabilities
• Updated Worker.Grpc.Tests.csproj to reference AzureBlobPayloads project

Test Pattern

[Fact]
public void UseExternalizedPayloads_AddsLargePayloadsCapability()
{
    ServiceCollection services = new();
    DefaultDurableTaskWorkerBuilder builder = new("test", services);
    
    builder.UseGrpc(opt => opt.Channel = channel);
    services.AddSingleton<PayloadStore>(new FakePayloadStore());
    
    builder.UseExternalizedPayloads();
    
    var options = provider.GetService<IOptionsMonitor<GrpcDurableTaskWorkerOptions>>()!.Get("test");
    options.Capabilities.Should().Contain(P.WorkerCapability.LargePayloads);
}

Follows existing test patterns for worker builder extension methods (e.g., UseScheduledTasks).

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