Hub top list returns 500 when entityType is missing

### Feature Summary

The hub top-list endpoint passes a missing `entityType` into the entity table selector. The resulting match error is exposed as a 500 response instead of rejecting the missing required parameter.

Before: missing `entityType` returns 500.

Expected: missing `entityType` returns 400, while valid entity types keep working.

Reproduction evidence:

1. Launch Texera from current `main`.
2. Request `GET /api/hub/getTops` without `entityType`.
3. Observe a 500 response.
4. Request `GET /api/hub/getTops?entityType=workflow&limit=1` and observe a 200 response.

Version and commit evidence:

1.3.0-incubating-SNAPSHOT (main)

**Commit Hash (Optional)**

98588bf2ab49aac65ebf002496eb057670e6fc07

**What browsers are you seeing the problem on?**

Not browser-specific.

**Relevant log output**

The missing parameter request returns HTTP 500 with a generated server error ID.

### Proposed Solution or Design

Expected: missing `entityType` returns 400, while valid entity types keep working.

### Affected Area

Workflow Engine (Amber)