Add configurable strict mode for MCP server initialization (#3)

Adds a Configuration struct to control server behavior around client initialization.
When strict mode is enabled, server enforces MCP spec requiring clients to initialize
before sending requests. Default mode is more permissive for non-compliant clients.

Sources/MCP/Server/Server.swift

@@ -8,6 +8,25 @@ import class Foundation.JSONEncoder
 
 /// Model Context Protocol server
 public actor Server {
+    /// The server configuration
+    public struct Configuration: Hashable, Codable, Sendable {
+        /// The default configuration.
+        public static let `default` = Configuration(strict: false)
+
+        /// The strict configuration.
+        public static let strict = Configuration(strict: true)
+
+        /// When strict mode is enabled, the server:
+        /// - Requires clients to send an initialize request before any other requests
+        /// - Rejects all requests from uninitialized clients with a protocol error
+        ///
+        /// While the MCP specification requires clients to initialize the connection
+        /// before sending other requests, some implementations may not follow this.
+        /// Disabling strict mode allows the server to be more lenient with non-compliant
+        /// clients, though this may lead to undefined behavior.
+        public var strict: Bool
+    }
+
     /// Implementation information
     public struct Info: Hashable, Codable, Sendable {
         /// The server name
@@ -110,7 +129,9 @@ public actor Server {
     /// The server version
     public nonisolated var version: String { serverInfo.version }
     /// The server capabilities
-    public var capabilities = Capabilities()
+    public var capabilities: Capabilities
+    /// The server configuration
+    public var configuration: Configuration
 
     /// Request handlers
     private var methodHandlers: [String: RequestHandlerBox] = [:]
@@ -133,10 +154,12 @@ public actor Server {
     public init(
         name: String,
         version: String,
-        capabilities: Server.Capabilities = .init()
+        capabilities: Server.Capabilities = .init(),
+        configuration: Configuration = .default
     ) {
         self.serverInfo = Server.Info(name: name, version: version)
         self.capabilities = capabilities
+        self.configuration = configuration
     }
 
     /// Start the server
@@ -274,13 +297,15 @@ public actor Server {
                 "id": "\(request.id)",
             ])
 
-        // The client SHOULD NOT send requests other than pings
-        // before the server has responded to the initialize request.
-        switch request.method {
-        case Initialize.name, Ping.name:
-            break
-        default:
-            try checkInitialized()
+        if configuration.strict {
+            // The client SHOULD NOT send requests other than pings
+            // before the server has responded to the initialize request.
+            switch request.method {
+            case Initialize.name, Ping.name:
+                break
+            default:
+                try checkInitialized()
+            }
         }
 
         // Find handler for method name
@@ -308,9 +333,11 @@ public actor Server {
             "Processing notification",
             metadata: ["method": "\(message.method)"])
 
-        // Check initialization state unless this is an initialized notification
-        if message.method != InitializedNotification.name {
-            try checkInitialized()
+        if configuration.strict {
+            // Check initialization state unless this is an initialized notification
+            if message.method != InitializedNotification.name {
+                try checkInitialized()
+            }
         }
 
         // Find notification handlers for this method