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.yml

@@ -28,6 +28,8 @@ queries:
     file_path: infrahub/transforms/queries/vrf_intent.gql
   - name: mlag_intent
     file_path: infrahub/transforms/queries/mlag_intent.gql
+  - name: bgp_intent
+    file_path: infrahub/transforms/queries/bgp_intent.gql
 
 jinja2_transforms:
   - name: vlan_yang_transform
@@ -50,3 +52,7 @@ jinja2_transforms:
     description: "Generate MLAG configuration payload from Infrahub intent"
     query: mlag_intent
     template_path: infrahub/transforms/templates/mlag_yang.j2
+  - name: bgp_yang_transform
+    description: "Generate BGP configuration payload from Infrahub intent"
+    query: bgp_intent
+    template_path: infrahub/transforms/templates/bgp_yang.j2