chore(server/mcp): update to accept other json content type request (googleapis#1049)

Toolbox MCP endpoint to accept request of multiple content type
according to the json schema
(https://www.jsonrpc.org/historical/json-rpc-over-http.html#http-header)

Toolbox endpoints only accept `Content-Type: application/json`. Update
to accept `Content-Type: application/json-rpc` and
`Content-Type:application/jsonrequest` as well.

Fixes googleapis#1004

Author: Yuan325

internal/server/mcp.go

@@ -214,7 +214,7 @@ func (s *stdioSession) write(ctx context.Context, response any) error {
 func mcpRouter(s *Server) (chi.Router, error) {
 	r := chi.NewRouter()
 
-	r.Use(middleware.AllowContentType("application/json"))
+	r.Use(middleware.AllowContentType("application/json", "application/json-rpc", "application/jsonrequest"))
 	r.Use(middleware.StripSlashes)
 	r.Use(render.SetContentType(render.ContentTypeJSON))