Vertex AI Agent Engine sandbox `:execute` returning 404 NOT_FOUND in us-central1

## Summary

Since 2026-04-28 ~18:51 UTC, calls to the Vertex AI Agent Engine sandbox `:execute` endpoint return `404 NOT_FOUND` ("Requested entity was not found.") for newly created sandboxes in `us-central1`. The sandbox is verified to be in `STATE_RUNNING` immediately before the failing call. Affects both production and staging projects, consistent with a server-side rollout.

## Versions

- `google-cloud-aiplatform==1.133.0`
- `google-genai==1.73.1`
- `google-adk==1.29.0`
- Python 3.11

## Repro

`client.agent_engines.sandboxes.execute_code(...)` posts:

```http
POST https://us-central1-aiplatform.googleapis.com/v1//:execute
Content-Type: application/json

{ "inputs": [{ "mimeType": "application/json",
"data": "" }] }
```

Manual probing confirms `:execute` is reachable and validates input:

| Body | HTTP |
|---|---|
| `{"inputs":[{"mimeType":"application/json","data":<...>}]}` (current SDK shape) | **404** |
| `{"inputs":[{"mimeType":"text/x-python","data":<...>}]}` | 400 INVALID_ARGUMENT |
| `{"inputs":[{"mimeType":"text/plain","data":<...>}]}` | 400 INVALID_ARGUMENT |
| `{"inputData": ...}` (legacy) | 400, "Unknown name 'inputData'" |
| `{"code": "..."}` | 400, "Unknown name 'code'" |
| Legacy `:executeCode` | 404 (empty body) |

## Additional findings

1. **Race / propagation ruled out.** Re-running with a 10s wait after `STATE_RUNNING` reproduces the same 404.
2. **`GET `** returns only `name, displayName, createTime, updateTime, state, spec, expireTime`. No `connectionInfo` field. Per commit [`677b55e`](https://github.com/googleapis/python-aiplatform/commit/677b55e), `SandboxEnvironmentConnectionInfo` is where `routing_token` lives in the new schema. The server isn't returning that block, so no client (old or new) has anything to forward.
3. **HEAD-of-`main` SDK reproduces the same 404.** Built python-aiplatform from `main` at HEAD (commit `0bb2ecd`, past `677b55e`), called `execute_code()` against a fresh staging sandbox in `STATE_RUNNING`. Result was identical: `ClientError 404 NOT_FOUND. {'error': {'code': 404, 'message': 'Requested entity was not found.', 'status': 'NOT_FOUND'}}`. The HEAD SDK still emits the `mimeType=application/json` shape because the server never populates `connectionInfo`.
4. The reproducer ran as the same service account our live staging Cloud Run service runs as, ruling out an IAM artefact specific to a probing identity.

## Hypothesis

A server-side rollout to `:execute` in `us-central1` on or around 2026-04-28T18:00 UTC changed the contract to require routing information that the API does not currently expose on the sandbox `GET` payload. The fix may need to ship server-side.

## Asks

1. Can you confirm whether a server-side change to Agent Engine sandbox `:execute` was rolled out in `us-central1` on/around 2026-04-28T18:00 UTC.
3. Any interim server-side workaround / route still accepting the previous shape?

Tracked internally on Google IssueTracker [507490536](https://issuetracker.google.com/issues/507490536).