Restore the reconnect-during-teardown cancel fast path

The collapse of the CONNECTING branch dropped a real race fix: HA
re-requesting a connect while a scheduled teardown was still pending
used to cancel the teardown and let the in-flight open complete;
without it the request was ignored and HA paid a teardown plus a fresh
connect during exactly the reconnect churn this path sees most.

cancel_gatt_disconnect() joins the contract: true only for a scheduled
teardown that has not started closing (Bluedroid clears the latched
want_disconnect_); rp2 and the stubs return false since their teardowns
start inside gatt_disconnect(). The wrapper's cancel_teardown() returns
its state to CONNECTING and the proxy handler carries the dev branch
verbatim.

tests/components/ble_device_base/test_gatt_client_contract.cpp

@@ -31,6 +31,7 @@ class MinimalConnection {
     this->listener_->on_connection_state(true, 517, 0);
     return 0;
   }
+  bool cancel_gatt_disconnect() { return false; }
   int gatt_disconnect() { return 0; }
   int discover_services() {
     this->listener_->on_service_discovery_done(0);