Add server initialization hook (#19)

Tests/MCPTests/ServerTests.swift

@@ -48,4 +48,89 @@ struct ServerTests {
         await server.stop()
         await transport.disconnect()
     }
+
+    @Test("Initialize hook - successful")
+    func testInitializeHookSuccess() async throws {
+        let transport = MockTransport()
+
+        actor TestState {
+            var hookCalled = false
+            func setHookCalled() { hookCalled = true }
+            func wasHookCalled() -> Bool { hookCalled }
+        }
+
+        let state = TestState()
+        let server = Server(name: "TestServer", version: "1.0")
+
+        // Start with the hook directly
+        try await server.start(transport: transport) { clientInfo, capabilities in
+            #expect(clientInfo.name == "TestClient")
+            #expect(clientInfo.version == "1.0")
+            await state.setHookCalled()
+        }
+        
+        // Wait for server to initialize
+        try await Task.sleep(nanoseconds: 10_000_000)  // 10ms
+
+        // Queue an initialize request
+        try await transport.queueRequest(
+            Initialize.request(
+                .init(
+                    protocolVersion: Version.latest,
+                    capabilities: .init(),
+                    clientInfo: .init(name: "TestClient", version: "1.0")
+                )
+            ))
+
+        // Wait for message processing and hook execution
+        try await Task.sleep(nanoseconds: 200_000_000)  // 200ms
+
+        #expect(await state.wasHookCalled() == true)
+        #expect(await transport.sentMessages.count >= 1)
+
+        let messages = await transport.sentMessages
+        if let response = messages.first {
+            #expect(response.contains("serverInfo"))
+        }
+
+        await server.stop()
+    }
+
+    @Test("Initialize hook - rejection")
+    func testInitializeHookRejection() async throws {
+        let transport = MockTransport()
+
+        let server = Server(name: "TestServer", version: "1.0")
+
+        try await server.start(transport: transport) { clientInfo, _ in
+            if clientInfo.name == "BlockedClient" {
+                throw Error.invalidRequest("Client not allowed")
+            }
+        }
+        
+        // Wait for server to initialize
+        try await Task.sleep(nanoseconds: 10_000_000)  // 10ms
+
+        // Queue an initialize request from blocked client
+        try await transport.queueRequest(
+            Initialize.request(
+                .init(
+                    protocolVersion: Version.latest,
+                    capabilities: .init(),
+                    clientInfo: .init(name: "BlockedClient", version: "1.0")
+                )
+            ))
+
+        // Wait for message processing
+        try await Task.sleep(nanoseconds: 200_000_000)  // 200ms
+
+        #expect(await transport.sentMessages.count >= 2)
+
+        let messages = await transport.sentMessages
+        if let response = messages.first {
+            #expect(response.contains("error"))
+            #expect(response.contains("Client not allowed"))
+        }
+        await server.stop()
+    }
 }