feat(compute): probe uBridge's swappable bridge TAP leg

The bridge module gained delete_nio_tap (uBridge's gns3server-integration
contract, section 'bridge — generic NIO relay with a swappable TAP leg'):
a per-port bridge can swap its topology leg between UDP and a persistent
TAP through stop -> delete -> add -> start, which is what lets an IOL
runner container's port bridge carry a kernel-link anchor.

The probe needs no scratch objects and no capabilities: one command
against a bridge that cannot exist — 202 (unknown command) is an old
build, 214 (bridge doesn't exist) is a new one. Cached by binary identity
like every other probe; any failure reports unknown.

GET /v3/compute/capabilities gains ubridge_bridge_tap; the controller's
default capability dict carries it too (the single schema definition, per
the 1e67fbf12 lesson).

gns3server/controller/compute.py

@@ -102,6 +102,7 @@ class Compute:
             "ubridge_tc": None,
             "ubridge_tap": None,
             "ubridge_iol_tap": None,
+            "ubridge_bridge_tap": None,
         }
         self.name = name
         # Cache of interfaces on remote host