fix(rpc): fix seer RPC exception handling to default to 500 and preserve rest APIException (#112435)

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## Problem

When Seer makes RPC calls to Sentry and encounters downstream errors
(like 429 rate limits or 500 errors from Snuba), the current code
catches all exceptions and re-raises them as `ValidationError` (400).
This misrepresents the actual issue, making it appear as if the client
sent bad request parameters when the real cause is a downstream service
error.

Example issue:
https://sentry.dev.getsentry.net:7999/issues/7380803020/events/d3e6e1d5ddf44d7b94c86e6ce2435ea6/

## Solution

This PR fixes the exception handling in the seer RPC endpoint to raise a
generic 500 APIException by default, implying an error happened on
sentry server. Also re-raises APIException's (base class for rest
framework excs) so developers can bubble their own status codes up to
response for Seer.

1. **Added a handler to re-raise REST framework exceptions directly** -
Any `APIException` (which includes `Throttled`, custom API exceptions,
etc.) is now re-raised as-is, preserving its status code
2. **Changed generic exception handler to raise `APIException` (500)
instead of `ValidationError` (400)** - Unknown errors now correctly
return 500 Internal Server Error instead of 400 Bad Request

### Changes

- Modified `SeerRpcServiceEndpoint.post()` in
`src/sentry/seer/endpoints/seer_rpc.py`:
- Added `except APIException: raise` handler before the generic
exception handler
- Changed `raise ValidationError from e` to `raise APIException from e`
for generic exceptions
- Added test cases in `tests/sentry/seer/endpoints/test_seer_rpc.py`:
- `test_rest_framework_exceptions_are_reraised` - verifies custom API
exceptions preserve their status codes
- `test_generic_exceptions_return_500` - verifies generic exceptions
return 500 instead of 400

## Testing

Pre-commit checks pass ✅

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---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Andrew Liu <aliu39@users.noreply.github.com>

Author: aliu39

src/sentry/seer/endpoints/seer_rpc.py

@@ -271,13 +271,15 @@ def post(self, request: Request, method_name: str) -> Response:
         except SnubaRPCRateLimitExceeded as e:
             sentry_sdk.capture_exception()
             raise Throttled(detail="Rate limit exceeded") from e
+        except APIException:
+            raise
         except Exception as e:
             if in_test_environment():
                 raise
             if settings.DEBUG:
                 raise Exception(f"Problem processing seer rpc endpoint {method_name}") from e
             sentry_sdk.capture_exception()
-            raise ValidationError from e
+            raise APIException from e
         return Response(data=result)
 
 

tests/sentry/seer/endpoints/test_seer_rpc.py

@@ -87,6 +87,50 @@ def test_snuba_rate_limit_returns_429(self) -> None:
         assert response.status_code == 429
         assert "Rate limit exceeded" in response.data["detail"]
 
+    def test_rest_framework_exceptions_are_reraised(self) -> None:
+        """Test that REST framework exceptions preserve their status codes."""
+        from rest_framework.exceptions import APIException
+
+        class CustomAPIException(APIException):
+            status_code = 503
+            default_detail = "Service temporarily unavailable"
+
+        path = self._get_path("get_organization_slug")
+        data: dict[str, Any] = {"args": {"org_id": 1}, "meta": {}}
+
+        with patch(
+            "sentry.seer.endpoints.seer_rpc.SeerRpcServiceEndpoint._dispatch_to_local_method"
+        ) as mock_dispatch:
+            mock_dispatch.side_effect = CustomAPIException()
+
+            response = self.client.post(
+                path, data=data, HTTP_AUTHORIZATION=self.auth_header(path, data)
+            )
+
+        assert response.status_code == 503
+        assert "Service temporarily unavailable" in response.data["detail"]
+
+    def test_generic_exceptions_return_500(self) -> None:
+        """Test that generic exceptions return 500 instead of 400."""
+        path = self._get_path("get_organization_slug")
+        data: dict[str, Any] = {"args": {"org_id": 1}, "meta": {}}
+
+        for is_test_environment in [True, False]:
+            with patch(
+                "sentry.seer.endpoints.seer_rpc.in_test_environment",
+                return_value=is_test_environment,
+            ):
+                with patch(
+                    "sentry.seer.endpoints.seer_rpc.SeerRpcServiceEndpoint._dispatch_to_local_method"
+                ) as mock_dispatch:
+                    mock_dispatch.side_effect = RuntimeError("Unexpected internal error")
+
+                    response = self.client.post(
+                        path, data=data, HTTP_AUTHORIZATION=self.auth_header(path, data)
+                    )
+
+                assert response.status_code == 500
+
 
 class TestSeerRpcMethods(APITestCase):
     """Test individual RPC methods"""