Add integration tests and fix test forward-compatibility

Add McpClient + Stateful Streamable HTTP integration tests for MRTR,
and a test verifying experimental protocol version negotiation. Fix the
sampling log assertion in MapMcpTests to handle MRTR mode where sampling
is embedded in the IncompleteResult exchange. Demote MrtrHandlerError
from Error to Debug to avoid double-logging since the observer just
confirms the exception was observed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: halter73

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -1732,7 +1732,7 @@ private async Task TrackDeferredHandlerTaskAsync(
     [LoggerMessage(Level = LogLevel.Debug, Message = "Cancelled {Count} pending MRTR continuation(s) during session disposal.")]
     private partial void MrtrContinuationsCancelled(int count);
 
-    [LoggerMessage(Level = LogLevel.Error, Message = "An MRTR handler threw an unhandled exception.")]
+    [LoggerMessage(Level = LogLevel.Debug, Message = "An MRTR handler threw an unhandled exception.")]
     private partial void MrtrHandlerError(Exception exception);
 
     /// <summary>

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs

@@ -197,9 +197,17 @@ public async Task Sampling_DoesNotCloseStreamPrematurely()
             m.Category == "ModelContextProtocol.Client.McpClient" &&
             m.Message.Contains("request '2' for method 'tools/call'"));
 
-        Assert.Single(MockLoggerProvider.LogMessages, m =>
+        // In MRTR mode, sampling is embedded in the IncompleteResult exchange within tools/call,
+        // so there's no separate sampling/createMessage JSON-RPC request logged by the server.
+        bool hasSeparateSamplingRequest = MockLoggerProvider.LogMessages.Any(m =>
             m.Category == "ModelContextProtocol.Server.McpServer" &&
-            m.Message.Contains("request '2' for method 'sampling/createMessage'"));
+            m.Message.Contains("for method 'sampling/createMessage'"));
+        if (hasSeparateSamplingRequest)
+        {
+            Assert.Single(MockLoggerProvider.LogMessages, m =>
+                m.Category == "ModelContextProtocol.Server.McpServer" &&
+                m.Message.Contains("request '2' for method 'sampling/createMessage'"));
+        }
     }
 
     [Fact]