Replace wildcard pattern with explicit exhaustive pattern

Changed the default wildcard case `_` to an explicit pattern
`{ HasId: false, HasMethod: false }` to make the switch expression
exhaustive and improve code clarity. All boolean combinations of
HasId and HasMethod are now explicitly covered.

Co-authored-by: Scooletz <scooletz@users.noreply.github.com>
Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/JsonRpcMessage.cs

@@ -124,8 +124,8 @@ public sealed class Converter : JsonConverter<JsonRpcMessage>
                     Params = union.Params
                 },
 
-                // All other cases are invalid
-                _ => throw new JsonException("Invalid JSON-RPC message format")
+                // Messages with neither id nor method are invalid
+                { HasId: false, HasMethod: false } => throw new JsonException("Invalid JSON-RPC message format")
             };
         }