feat: Add Infrahub Jinja2 transform for BGP configuration (#23)

Single unified bgp_yang_transform covering the full BGP router stanza
(process config, peer groups, neighbors, address families) for all
10 fabric devices in a single query+template pass.

- Add GraphQL query bgp_intent.gql: queries InfraBGPRouterConfig by
  device name, traversing local_asn, peer_groups (with remote_asn),
  and sessions (with peer_group, remote_asn, vrf, peer_device);
  separately queries InfraBGPAddressFamily for the same device,
  including active_peer_groups, active_sessions, networks, and optional
  vrf relationship
- Add Jinja2 template bgp_yang.j2: renders a JSON object with bgp.global
  (ASN, router-id, distances, ECMP), bgp.peer_groups, bgp.neighbors,
  bgp.address_families for global sessions, plus bgp.vrf_neighbors and
  bgp.vrf_address_families for VRF-scoped entries (e.g. leaf7 VRF gold
  border peering); returns [] for devices with no BGP config; all
  optional relationships guarded with 'is defined and is not none' checks;
  send_community "none" normalised to null in output
- Update .infrahub.yml: register bgp_intent query and bgp_yang_transform
- Add unit test fixtures for three representative devices:
    leaf1  — 3 peer-groups, 5 global neighbors, 2 global AFs
    spine1 — 1 peer-group (evpn/next-hop-unchanged), 16 neighbors
             (8 direct underlay with remote_asn, 8 EVPN via peer-group),
             IPv4 AF activates individual sessions
    leaf7  — leaf1 pattern + VRF gold border session (AS 64999) and
             VRF-scoped IPv4 unicast AF

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infrahub/transforms/tests/bgp_yang/leaf7/output.json

@@ -0,0 +1,140 @@
+{
+  "bgp": {
+    "global": {
+      "asn": 65004,
+      "router_id": "10.0.250.17",
+      "default_ipv4_unicast": false,
+      "log_neighbor_changes": true,
+      "distance": {
+        "ebgp": 20,
+        "ibgp": 200,
+        "local": 200
+      },
+      "ecmp": {
+        "max_paths": 4,
+        "max_ecmp": 64
+      }
+    },
+    "peer_groups": [
+      {
+        "name": "underlay",
+        "type": "underlay",
+        "remote_asn": 65000,
+        "update_source": null,
+        "ebgp_multihop": null,
+        "send_community": null,
+        "next_hop_self": false,
+        "next_hop_unchanged": false,
+        "maximum_routes": 12000,
+        "maximum_routes_warning_only": true
+      },
+      {
+        "name": "underlay_ibgp",
+        "type": "underlay_ibgp",
+        "remote_asn": 65004,
+        "update_source": null,
+        "ebgp_multihop": null,
+        "send_community": null,
+        "next_hop_self": true,
+        "next_hop_unchanged": false,
+        "maximum_routes": 12000,
+        "maximum_routes_warning_only": true
+      },
+      {
+        "name": "evpn",
+        "type": "evpn",
+        "remote_asn": 65000,
+        "update_source": "Loopback0",
+        "ebgp_multihop": 3,
+        "send_community": "extended",
+        "next_hop_self": false,
+        "next_hop_unchanged": false,
+        "maximum_routes": 12000,
+        "maximum_routes_warning_only": true
+      }
+    ],
+    "neighbors": [
+      {
+        "peer_address": "10.0.1.12",
+        "description": "underlay to spine1",
+        "enabled": true,
+        "peer_group": "underlay",
+        "remote_asn": null
+      },
+      {
+        "peer_address": "10.0.2.12",
+        "description": "underlay to spine2",
+        "enabled": true,
+        "peer_group": "underlay",
+        "remote_asn": null
+      },
+      {
+        "peer_address": "10.0.3.7",
+        "description": "iBGP to leaf8",
+        "enabled": true,
+        "peer_group": "underlay_ibgp",
+        "remote_asn": null
+      },
+      {
+        "peer_address": "10.0.250.1",
+        "description": "EVPN to spine1",
+        "enabled": true,
+        "peer_group": "evpn",
+        "remote_asn": null
+      },
+      {
+        "peer_address": "10.0.250.2",
+        "description": "EVPN to spine2",
+        "enabled": true,
+        "peer_group": "evpn",
+        "remote_asn": null
+      }
+    ],
+    "address_families": [
+      {
+        "afi": "ipv4",
+        "safi": "unicast",
+        "active_peer_groups": [
+          "underlay",
+          "underlay_ibgp"
+        ],
+        "active_sessions": [],
+        "networks": [
+          "10.0.250.17/32",
+          "10.0.255.14/32"
+        ]
+      },
+      {
+        "afi": "evpn",
+        "safi": "unicast",
+        "active_peer_groups": [
+          "evpn"
+        ],
+        "active_sessions": [],
+        "networks": []
+      }
+    ],
+    "vrf_neighbors": [
+      {
+        "peer_address": "10.90.90.1",
+        "description": "border peering to AS 64999 in VRF gold",
+        "enabled": true,
+        "peer_group": null,
+        "remote_asn": 64999,
+        "vrf": "gold"
+      }
+    ],
+    "vrf_address_families": [
+      {
+        "afi": "ipv4",
+        "safi": "unicast",
+        "vrf": "gold",
+        "active_peer_groups": [],
+        "active_sessions": [
+          "10.90.90.1"
+        ],
+        "networks": []
+      }
+    ]
+  }
+}