Flow ExecutionContext with JsonRpcMessage (#616)

The primary goal of this change is to support IHttpContextAccessor in tool calls when the Streamable HTTP is in its default non-Stateless mode.

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs

@@ -52,12 +52,6 @@ public abstract class MapMcpTests(ITestOutputHelper testOutputHelper) : KestrelI
     [Fact]
     public async Task Can_UseIHttpContextAccessor_InTool()
     {
-        Assert.SkipWhen(UseStreamableHttp && !Stateless,
-            """
-            IHttpContextAccessor is not currently supported with non-stateless Streamable HTTP.
-            TODO: Support it in stateless mode by manually capturing and flowing execution context.
-            """);
-
         Builder.Services.AddMcpServer().WithHttpTransport(ConfigureStateless).WithTools<EchoHttpContextUserTools>();
 
         Builder.Services.AddHttpContextAccessor();