Fix a latent bug in DAP request decorator

The 'request' decorator is intended to also ensure that the request
function runs in the DAP thread.  However, the unwrapped function is
installed in the global request map, so the wrapped version is never
called.  This patch fixes the bug.

gdb/python/lib/gdb/dap/server.py

@@ -164,9 +164,10 @@ def request(name):
 
     def wrap(func):
         global _commands
-        _commands[name] = func
         # All requests must run in the DAP thread.
-        return in_dap_thread(func)
+        func = in_dap_thread(func)
+        _commands[name] = func
+        return func
 
     return wrap