feat: Add Infrahub Jinja2 transform for VRF/L3VNI configuration (#21) (#25)

## Summary

Implements Infrahub Jinja2 Transform for VRF/L3VNI configuration, generating structured JSON payloads from Infrahub intent data. Closes #21.

## What's included

- **GraphQL query** (`transforms/queries/vrf_intent.gql`): Queries `InfraVRFDeviceAssignment` by device name, fetching VRF details, L3VNI, route targets, and device-specific route distinguisher
- **Jinja2 transform** (`transforms/templates/vrf_yang.j2`): Renders VRF config payloads with defensive null handling (lessons learned from #20)
- **`.infrahub.yml`** updated with the new query and transform definitions
- **Tests** (`transforms/tests/vrf_yang/`) using correct Infrahub pytest format (`jinja2-transform-smoke` + `jinja2-transform-unit-render`)

## Validation

All transforms render correctly:

| Device | Expected | Result |
|--------|----------|--------|
| `leaf3` | VRF gold, RD `10.0.250.13:1`, L3VNI 100001 | ✅ |
| `leaf7` | VRF gold, RD `10.0.250.17:1`, L3VNI 100001 | ✅ |
| `leaf1` | Empty array (no VRF assignment) | ✅ |
| `spine1` | Empty array (no VRF assignment) | ✅ |

## Usage

```bash
# Local debugging
infrahubctl render vrf_yang_transform device_name=leaf3

# API endpoint
GET /api/transform/jinja2/vrf_yang_transform?device_name=leaf3
```

## Related

- Depends on: #20 (base transforms infrastructure, already merged)
- Reference: Arista EVPN Type-5 / L3VXLAN configuration from lab topology

infrahub/transforms/tests/vrf_yang/input.json

@@ -0,0 +1,82 @@
+{
+  "data": {
+    "InfraVRFDeviceAssignment": {
+      "edges": [
+        {
+          "node": {
+            "route_distinguisher": {
+              "value": "10.0.250.13:1"
+            },
+            "vrf": {
+              "node": {
+                "name": {
+                  "value": "gold"
+                },
+                "description": {
+                  "value": "VRF gold - L3 VXLAN with symmetric IRB"
+                },
+                "route_distinguisher": {
+                  "value": null
+                },
+                "l3vni": {
+                  "node": {
+                    "vni": {
+                      "value": 100001
+                    },
+                    "vni_type": {
+                      "value": "l3vni"
+                    }
+                  }
+                },
+                "import_targets": {
+                  "edges": [
+                    {
+                      "node": {
+                        "target": {
+                          "value": "1:100001"
+                        }
+                      }
+                    }
+                  ]
+                },
+                "export_targets": {
+                  "edges": [
+                    {
+                      "node": {
+                        "target": {
+                          "value": "1:100001"
+                        }
+                      }
+                    }
+                  ]
+                }
+              }
+            },
+            "import_targets": {
+              "edges": [
+                {
+                  "node": {
+                    "target": {
+                      "value": "1:100001"
+                    }
+                  }
+                }
+              ]
+            },
+            "export_targets": {
+              "edges": [
+                {
+                  "node": {
+                    "target": {
+                      "value": "1:100001"
+                    }
+                  }
+                }
+              ]
+            }
+          }
+        }
+      ]
+    }
+  }
+}