Public workflow retrieval returns 500 when unavailable

### Feature Summary

The public-workflow endpoint dereferences the database result without checking whether a public workflow was found. Missing workflows and existing private workflows therefore return 500 instead of 404.

Before: unavailable public workflow lookup returns 500.

Expected: unavailable public workflow lookup returns 404, while a public workflow still returns 200.

Reproduction evidence:

1. Launch Texera from current `main`.
2. Request `GET /api/workflow/publicised/999999` and observe a 500 response.
3. Request the same path for an existing private workflow and observe another 500 response.
4. Request it for a public workflow 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 endpoint calls workflow getters after the public-only query returns no record.

### Proposed Solution or Design

Expected: unavailable public workflow lookup returns 404, while a public workflow still returns 200.

### Affected Area

Workflow Engine (Amber)