fix(iou): return 405 error for unsupported suspend operation

Fixed IOU node suspend API to return proper HTTP 405 Method Not Allowed
error instead of misleading 204 No Content response.

Changes:
- Added HTTPException import to iou_nodes.py
- Fixed suspend_iou_node route from /stop to /suspend (bug fix)
- Changed response from 204 No Content to 405 Method Not Allowed
- Added clear error message: "Suspend is not supported for IOU nodes"

This fix ensures clients receive explicit feedback when attempting to
suspend IOU nodes, which do not support suspend functionality.

Related issue: IOU nodes previously returned 404 when suspend was called
due to incorrect route registration.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

gns3server/api/routes/compute/iou_nodes.py

@@ -20,7 +20,7 @@ API routes for IOU nodes.
 
 import os
 
-from fastapi import APIRouter, WebSocket, Depends, Body, status
+from fastapi import APIRouter, WebSocket, Depends, Body, status, HTTPException
 from fastapi.encoders import jsonable_encoder
 from fastapi.responses import StreamingResponse
 from typing import Union
@@ -186,8 +186,7 @@ async def stop_iou_node(node: IOUVM = Depends(dep_node)) -> None:
 
 
 @router.post(
-    "/{node_id}/stop",
-    status_code=status.HTTP_204_NO_CONTENT,
+    "/{node_id}/suspend",
     dependencies=[Depends(compute_authentication)]
 )
 def suspend_iou_node(node: IOUVM = Depends(dep_node)) -> None:
@@ -196,7 +195,10 @@ def suspend_iou_node(node: IOUVM = Depends(dep_node)) -> None:
     Does nothing since IOU doesn't support being suspended.
     """
 
-    pass
+    raise HTTPException(
+        status_code=status.HTTP_405_METHOD_NOT_ALLOWED,
+        detail="Suspend is not supported for IOU nodes"
+    )
 
 
 @router.post(