Add test coverage

Author: mattt

Tests/MCPTests/HTTPClientTransportTests.swift

@@ -674,4 +674,45 @@ import Testing
             await client.disconnect()
         }
     }
+
+    @Test("Request modifier functionality", .httpClientTransportSetup)
+    func testRequestModifier() async throws {
+        let testEndpoint = URL(string: "https://api.example.com/mcp")!
+        let testToken = "test-bearer-token-12345"
+
+        let configuration = URLSessionConfiguration.ephemeral
+        configuration.protocolClasses = [MockURLProtocol.self]
+
+        // Create transport with requestModifier that adds Authorization header
+        let transport = HTTPClientTransport(
+            endpoint: testEndpoint,
+            configuration: configuration,
+            streaming: false,
+            requestModifier: { request in
+                var modifiedRequest = request
+                modifiedRequest.addValue("Bearer \(testToken)", forHTTPHeaderField: "Authorization")
+                return modifiedRequest
+            },
+            logger: nil
+        )
+
+        try await transport.connect()
+
+        let messageData = #"{"jsonrpc":"2.0","method":"test","id":5}"#.data(using: .utf8)!
+
+        await MockURLProtocol.requestHandlerStorage.setHandler {
+            [testEndpoint, testToken] (request: URLRequest) in
+            // Verify the Authorization header was added by the requestModifier
+            #expect(request.value(forHTTPHeaderField: "Authorization") == "Bearer \(testToken)")
+
+            // Return a successful response
+            let response = HTTPURLResponse(
+                url: testEndpoint, statusCode: 200, httpVersion: "HTTP/1.1",
+                headerFields: ["Content-Type": "application/json"])!
+            return (response, Data())
+        }
+
+        try await transport.send(messageData)
+        await transport.disconnect()
+    }
 #endif  // swift(>=6.1)