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/test.yml

@@ -0,0 +1,27 @@
+---
+version: "1.0"
+infrahub_tests:
+  - resource: Jinja2Transform
+    resource_name: bgp_yang_transform
+    tests:
+      - name: smoke_check
+        spec:
+          kind: jinja2-transform-smoke
+      - name: render_leaf1
+        spec:
+          kind: jinja2-transform-unit-render
+          directory: infrahub/transforms/tests/bgp_yang/leaf1
+          input: input.json
+          output: output.json
+      - name: render_spine1
+        spec:
+          kind: jinja2-transform-unit-render
+          directory: infrahub/transforms/tests/bgp_yang/spine1
+          input: input.json
+          output: output.json
+      - name: render_leaf7
+        spec:
+          kind: jinja2-transform-unit-render
+          directory: infrahub/transforms/tests/bgp_yang/leaf7
+          input: input.json
+          output: output.json