(core) Fix uploads test: stub getDocWorkerInfoOrSelfPrefix via Deps, not module export

Summary:
The fetchDoc test (from upstream grist-core #2344) stubbed a module
namespace export, which works with tsc's CJS output but fails with
esbuild's non-configurable export getters used in this repo's build.
Move the function into the existing Deps injection hook instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Test Plan: Test has been failing on master, and should now pass.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D5006