feat: Add Infrahub Jinja2 transform for MLAG configuration (#22)

- Add GraphQL query mlag_intent.gql: queries InfraMlagPeerConfig by
  device name, returning local IP, peer address, heartbeat peer IP,
  MLAG domain (domain-id, virtual-mac, heartbeat VRF, dual-primary
  detection settings, peer/iBGP VLANs, peer devices), local interface
  SVI name, and peer-link LAG name
- Add Jinja2 template mlag_yang.j2: renders a JSON object targeting
  Arista-native YANG path /arista-mlag-augments:mlag/config; returns
  empty array [] for devices with no MLAG config (spines); all optional
  relationship accesses guarded with 'is defined and is not none' checks
- Update .infrahub.yml: register mlag_intent query and mlag_yang_transform
- Add unit test fixtures for leaf1 (full MLAG config) and spine1 (empty
  response → []) following jinja2-transform-unit-render format

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

.infrahub.yml

@@ -26,6 +26,8 @@ queries:
     file_path: infrahub/transforms/queries/vxlan_intent.gql
   - name: vrf_intent
     file_path: infrahub/transforms/queries/vrf_intent.gql
+  - name: mlag_intent
+    file_path: infrahub/transforms/queries/mlag_intent.gql
 
 jinja2_transforms:
   - name: vlan_yang_transform
@@ -44,3 +46,7 @@ jinja2_transforms:
     description: "Generate VRF/L3VNI configuration payload from Infrahub intent"
     query: vrf_intent
     template_path: infrahub/transforms/templates/vrf_yang.j2
+  - name: mlag_yang_transform
+    description: "Generate MLAG configuration payload from Infrahub intent"
+    query: mlag_intent
+    template_path: infrahub/transforms/templates/mlag_yang.j2