Failed event definition save shows an unhelpful "FetchError" toast next to the real error message

Whenever saving an event definition is refused, the correct reason is shown in the inline banner, but a second notification appears at the same time carrying no usable information.

### Repro

Any one of these three produces it:

**A. Missing stream**
1. **Alerts > Event Definitions > Create event definition**, give it a title, click **Next**.
2. Condition type **Filter & Aggregation**, Search Query `http_method:POST`, and leave **Streams and Categories** empty.
3. Go to **Summary** and click **Create event definition**.

**B. Invalid tag**
1. As above, but select the Default Stream.
2. On **Additional Details**, add the tag `bad tag`.
3. Go to **Summary** and click **Create event definition**.

**C. Unknown MITRE ID** (needs Illuminate content installed)
1. As above, with the Default Stream selected.
2. On **Additional Details**, add the tactic/technique ID `T9999`.
3. Go to **Summary** and click **Create event definition**.

### Observed

The inline banner is correct and specific in all three cases. At the same time a toast appears in the corner:

> Could not save EventDefinition
> Saving EventDefinition failed with status: FetchError: There was an error fetching a resource: . Additional information: Not available

Note the empty text after "resource:" and the "Not available". The toast names no field and no reason.

### Expected

Either no toast, since the inline banner already explains the problem, or a toast that repeats the actual reason.

Version: Graylog 7.2.0-beta.1

Start by reproducing the event-definition save flow using the missing-stream, invalid-tag, or unknown-MITRE-ID cases, then trace the save error handling and notification path. Done means the specific inline error remains visible and the additional toast is either absent or repeats a useful reason.

Written by the indexing model from the issue text.