fix(backend/llm-admin-api): fix usage count, slug mismatch, and recommended model guard

db_write.py:
- Add _node_model_value() helper: strips provider prefix from slug so
  queries match the enum value stored in AgentNode.constantInput
  (e.g. 'openai/gpt-4o' -> 'gpt-4o')
- get_model_usage: use _node_model_value(slug) — was always returning 0
- toggle_model_with_migration: use _node_model_value for both the SELECT
  (finding nodes) and the UPDATE (writing the replacement value)

admin_routes.py:
- toggle_model: guard against disabling the recommended model with 400
  'Cannot disable the recommended model. Change it first.'

autogpt_platform/backend/backend/server/v2/llm/admin_routes.py

@@ -302,16 +302,26 @@ async def toggle_model(
                 status_code=404, detail=f"Model with slug '{slug}' not found"
             )
 
+        is_enabling = request.get("is_enabled", True)
+        if not is_enabling and existing.isRecommended:
+            raise HTTPException(
+                status_code=400,
+                detail=(
+                    "Cannot disable the recommended model. "
+                    "Change the recommended model before disabling this one."
+                ),
+            )
+
         result = await db_write.toggle_model_with_migration(
             model_id=existing.id,
-            is_enabled=request.get("is_enabled", True),
+            is_enabled=is_enabling,
             migrate_to_slug=request.get("migrate_to_slug"),
             migration_reason=request.get("migration_reason"),
             custom_credit_cost=request.get("custom_credit_cost"),
         )
         await db_write.refresh_runtime_caches()
         logger.info(
-            f"Toggled model '{slug}' enabled={request.get('is_enabled')} "
+            f"Toggled model '{slug}' enabled={is_enabling} "
             f"(migrated {result['nodes_migrated']} nodes)"
         )
         return result

autogpt_platform/backend/backend/server/v2/llm/db_write.py

@@ -14,6 +14,15 @@ from backend.data.db import transaction
 logger = logging.getLogger(__name__)
 
 
+def _node_model_value(slug: str) -> str:
+    """Extract the model value stored in AgentNode.constantInput from a registry slug.
+
+    Registry slugs are formatted as 'provider/model-name' (e.g. 'openai/gpt-4o').
+    The LLM block stores only the model-name part (e.g. 'gpt-4o') in constantInput.
+    """
+    return slug.split("/", 1)[-1] if "/" in slug else slug
+
+
 def _build_provider_data(
     name: str,
     display_name: str,
@@ -293,13 +302,14 @@ async def get_model_usage(slug: str) -> dict[str, Any]:
     """Get usage count for a model — how many AgentNodes reference it."""
     import prisma as prisma_module
 
+    model_value = _node_model_value(slug)
     count_result = await prisma_module.get_client().query_raw(
         """
         SELECT COUNT(*) as count
         FROM "AgentNode"
         WHERE "constantInput"::jsonb->>'model' = $1
         """,
-        slug,
+        model_value,
     )
     node_count = int(count_result[0]["count"]) if count_result else 0
     return {"model_slug": slug, "node_count": node_count}
@@ -337,6 +347,8 @@ async def toggle_model_with_migration(
                     f"Please enable it before using it as a replacement."
                 )
 
+            source_value = _node_model_value(model.slug)
+            target_value = _node_model_value(migrate_to_slug)
             node_ids_result = await tx.query_raw(
                 """
                 SELECT id
@@ -344,7 +356,7 @@ async def toggle_model_with_migration(
                 WHERE "constantInput"::jsonb->>'model' = $1
                 FOR UPDATE
                 """,
-                model.slug,
+                source_value,
             )
             migrated_node_ids = (
                 [row["id"] for row in node_ids_result] if node_ids_result else []
@@ -365,7 +377,7 @@ async def toggle_model_with_migration(
                         SELECT jsonb_array_elements_text($2::jsonb)
                     )
                     """,
-                    migrate_to_slug,
+                    target_value,
                     node_ids_json,
                 )