fix: add StatelessModeNotSupported exception and improve tests (#1828)

src/mcp/shared/exceptions.py

@@ -18,6 +18,24 @@ class McpError(Exception):
         self.error = error
 
 
+class StatelessModeNotSupported(RuntimeError):
+    """
+    Raised when attempting to use a method that is not supported in stateless mode.
+
+    Server-to-client requests (sampling, elicitation, list_roots) are not
+    supported in stateless HTTP mode because there is no persistent connection
+    for bidirectional communication.
+    """
+
+    def __init__(self, method: str):
+        super().__init__(
+            f"Cannot use {method} in stateless HTTP mode. "
+            "Stateless mode does not support server-to-client requests. "
+            "Use stateful mode (stateless_http=False) to enable this feature."
+        )
+        self.method = method
+
+
 class UrlElicitationRequiredError(McpError):
     """
     Specialized error for when a tool requires URL mode elicitation(s) before proceeding.