feat(infrahub): add Infrahub client for fabric intent (#42)

- Replace pynetbox with infrahub-sdk>=0.16.0 + pydantic>=2.0 in dependencies
- Add pytest and pytest-asyncio to dev dependencies
- Implement FabricInfrahubClient async client (src/infrahub/client.py)
  - get_device, get_device_vlans, get_device_bgp_config
  - get_device_bgp_peer_groups, get_device_bgp_sessions
  - get_device_vrfs, get_device_vtep, get_device_evpn_instances
  - get_mlag_domain, get_mlag_peer_config
  - TTL-based caching (60s) to avoid redundant SDK queries
  - Async context manager support
- Add Pydantic v2 frozen models for all intent types (src/infrahub/models.py)
- Add custom exception hierarchy (src/infrahub/exceptions.py)
- Add unit tests with fully mocked SDK (tests/test_infrahub_client.py)
  - Tests for correct model return, NotFoundError, branch selection, caching

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

pyproject.toml

@@ -1,13 +1,14 @@
 [project]
 name = "fabric-orchestrator"
 version = "0.1.0"
-description = "Declarative Network Fabric Orchestrator - Terraform-like infrastructure management for Arista EVPN-VXLAN using gNMI, YANG, and NetBox as Source of Truth"
+description = "Declarative Network Fabric Orchestrator - Terraform-like infrastructure management for Arista EVPN-VXLAN using gNMI, YANG, and Infrahub as Source of Truth"
 readme = "README.md"
 requires-python = ">=3.12"
 dependencies = [
     "click>=8.1.0",
     "pygnmi>=0.8.0",
-    "pynetbox>=7.5.0",
+    "infrahub-sdk>=0.16.0",
+    "pydantic>=2.0",
     "rich>=13.0.0",
 ]
 
@@ -35,5 +36,7 @@ ignore = ["E501"]
 [dependency-groups]
 dev = [
     "basedpyright>=1.37.4",
+    "pytest>=8.0.0",
+    "pytest-asyncio>=0.23.0",
     "ruff>=0.14.10",
 ]