fix(mcp): POST /mcp route (not /mcp/mcp)

basePath = '/mcp' + app.post("/") = POST /mcp
之前是 app.post("/mcp") 導致 /mcp/mcp 雙層，發請求到 /mcp 只得 404。

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

mcp/src/index.ts

@@ -233,7 +233,7 @@ app.options("/mcp", (c) => {
   });
 });
 
-app.post("/mcp", partnerAuthMiddleware, async (c) => {
+app.post("/", partnerAuthMiddleware, async (c) => {
   const orgNamespace = c.get("org_namespace");
   const partnerToken = c.get("partner_token");
   return handleMcpRequest(c.req.raw, c.env, orgNamespace, partnerToken);