fix: correct HealthCheckResponse status field type

Changed status from str to int to match actual API response.

**Test Results:**
- ✅ get_health() returns HealthCheckResponse correctly
- ✅ get_cdn() returns CDNResponse correctly
- ✅ get_demand() returns DemandResponse correctly
- ✅ get_demand_stream() returns List[InitialDemandOutturn] correctly

All manually created models validated against real API responses.

Author: BenjaminWatts

elexon_bmrs/untyped_models.py

@@ -24,7 +24,7 @@ class HealthCheckResponse(BaseModel):
     """Response from /health endpoint."""
     model_config = ConfigDict(extra='allow')
 
-    status: str
+    status: int  # Status code (e.g., 2 for healthy)
     total_duration: Optional[str] = Field(alias="totalDuration", default=None)
     entries: Optional[Dict[str, Any]] = None