fix(serve): return 404 status code when application or deployment not found (#51442) (#65584)

## Description
Makes the Serve dashboard scale endpoint report client errors with 404
status code instead of 400 when an application or deployment is not
found.

#51417 added the `HTTPStatusCode` enum and the
`rest_response(status_code=...)` plumbing, but adoption across dashboard
modules is ongoing.

**Backend (`python/ray/dashboard/modules/serve/serve_head.py`):**
- In `scale_deployment`, when `ValueError` with `"not found"` occurred
(looking up a nonexistent application or deployment), the endpoint
returned HTTP 400 (Bad Request). Scaling a nonexistent resource is a
client error representing a missing resource, so it now returns HTTP 404
(Not Found).

## Related issues

Related to #51442 (umbrella: revisit Ray dashboard API status codes).

Since #51442 is an umbrella issue, this PR covers the `serve_head.py`
scope only, allowing each dashboard module to be reviewed independently.

## Tests
Updated the existing unit test `test_error_case` in
`python/ray/dashboard/modules/serve/tests/test_serve_dashboard.py`:
- Verified that scaling a nonexistent application
(`app_name="nonexistent"`) returns HTTP status code 404 instead of 400.

## Not a duplicate
Checked existing open issues and PRs for #51442. No active PR currently
addresses status code fixes for `serve_head.py`. Commented on #51442
prior to working on this scope.

Signed-off-by: Vũ Hoàng Minh <vhminh23@clc.fitus.edu.vn>

Author: CrossManger

python/ray/dashboard/modules/serve/serve_head.py

@@ -264,7 +264,7 @@ async def scale_deployment(self, req: Request) -> Response:
                 return self._create_json_response({"error": str(e.cause)}, 412)
             if isinstance(e, ValueError) and "not found" in str(e):
                 return self._create_json_response(
-                    {"error": "Application or Deployment not found"}, 400
+                    {"error": "Application or Deployment not found"}, 404
                 )
             else:
                 logger.error(

python/ray/dashboard/modules/serve/tests/test_serve_dashboard.py

@@ -969,7 +969,7 @@ def test_error_case(self, ray_start_stop):
             json={"target_num_replicas": 2},
             timeout=30,
         )
-        assert error_response.status_code == 400
+        assert error_response.status_code == 404
         assert "not found" in error_response.json()["error"].lower()
 
         error_response = requests.post(