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/schemas/controller/capabilities.py

@@ -61,3 +61,9 @@ class Capabilities(BaseModel):
         "the anchor IOU Ethernet ports need to wire IOU links on the kernel datapath); absent when the "
         "probe failed",
     )
+    ubridge_bridge_tap: Optional[bool] = Field(
+        None,
+        description="Whether uBridge's generic bridge module can release a named TAP NIO "
+        "(bridge delete_nio_tap, the swappable TAP leg an IOL runner container's per-port bridge "
+        "needs to wire its links on the kernel datapath); absent when the probe failed",
+    )