Send HTTP/DELETE when disconnecting client

* In server/src/index.ts
  - add delete handler for /mcp endpoint
    - gets the server transport for the sessionId
    - calls terminateSession on it
    - removes the webapp and server transports for the sessionId from the maps
    - returns status 200 to the client.

* In client/src/lib/hooks/useConnection.ts
  - import Transport
  - add useState for clientTransport, type Transport or null initialized to null
  - in connect() function
    - move creation of client transport down a ways, just before calling client.connect with it
    - immendiately efter calling client.connect, call setClientTransport with the connected transport (has to happen after, so that what's saved has the abort controller, etc. otherwise it doesn't work to save it prior.
    - in disconnect() function
      - immediately call clientTransport.terminateSession if transportType is "streamable-http"
      - setClientTransport to null

server/src/index.ts

@@ -226,6 +226,33 @@ app.post("/mcp", async (req, res) => {
   }
 });
 
+app.delete("/mcp", async (req, res) => {
+  const sessionId = req.headers["mcp-session-id"] as string | undefined;
+  console.log(`Received DELETE message for sessionId ${sessionId}`);
+  let serverTransport: Transport | undefined;
+  if (sessionId) {
+    try {
+      serverTransport = serverTransports.get(
+        sessionId,
+      ) as StreamableHTTPClientTransport;
+      if (!serverTransport) {
+        res.status(404).end("Transport not found for sessionId " + sessionId);
+      } else {
+        await (
+          serverTransport as StreamableHTTPClientTransport
+        ).terminateSession();
+        webAppTransports.delete(sessionId);
+        serverTransports.delete(sessionId);
+        console.log(`Transports removed for sessionId ${sessionId}`);
+      }
+      res.status(200).end();
+    } catch (error) {
+      console.error("Error in /mcp route:", error);
+      res.status(500).json(error);
+    }
+  }
+});
+
 app.get("/stdio", async (req, res) => {
   try {
     console.log("New connection");