Dashboard search returns server error for an unknown resource type

### Feature Summary

The dashboard search dispatch throws an unhandled `IllegalArgumentException` for an unknown `resourceType`, so the API reports an internal server error for invalid client input.

Before: unknown resource type returns 500.
After: unknown resource type returns 400 with a specific message.

Reproduction evidence:

1. Start the local development backend from main.
2. Request `GET /api/dashboard/publicSearch?resourceType=bogus`.
3. Observe HTTP 500.
4. Request the same endpoint with `resourceType=workflow` and observe HTTP 200.

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 request is logged as an unhandled IllegalArgumentException with `Unknown resource type: bogus`.

### Proposed Solution or Design

After: unknown resource type returns 400 with a specific message.

### Affected Area

Workflow Engine (Amber)