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/transforms/tests/mlag_yang/test.yml

@@ -0,0 +1,21 @@
+---
+version: "1.0"
+infrahub_tests:
+  - resource: Jinja2Transform
+    resource_name: mlag_yang_transform
+    tests:
+      - name: smoke_check
+        spec:
+          kind: jinja2-transform-smoke
+      - name: render_leaf1
+        spec:
+          kind: jinja2-transform-unit-render
+          directory: infrahub/transforms/tests/mlag_yang/leaf1
+          input: input.json
+          output: output.json
+      - name: render_spine1
+        spec:
+          kind: jinja2-transform-unit-render
+          directory: infrahub/transforms/tests/mlag_yang/spine1
+          input: input.json
+          output: output.json