Dashboard search accepts negative pagination values

### Feature Summary

Dashboard search accepts negative pagination values instead of rejecting invalid client input.

Before: `start=-1` returns 500, and `count=-1` returns 200 with an inconsistent `more=true` response.
After: negative `start` or `count` returns 400, while zero remains valid.

Reproduction evidence:

1. Start the local development backend from main.
2. Request `GET /api/dashboard/publicSearch?resourceType=workflow&start=-1` and observe HTTP 500.
3. Request the endpoint with `count=-1` and observe HTTP 200 with an empty result and `more=true`.
4. Request the endpoint with `start=0&count=0` 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**

Negative offset reaches the database query instead of being rejected at the API boundary.

### Proposed Solution or Design

After: negative `start` or `count` returns 400, while zero remains valid.

### Affected Area

Workflow Engine (Amber)