Add regression test for server.signingWorker being fully optional

[ayushsingh82]

Summary

server.signingWorker (wallet-gateway/remote/src/config/Config.ts) is already wrapped in z.preprocess((val) => val ?? {}, ...), with pollInterval defaulting to 5000. That already makes the whole signingWorker block optional, with a sane default, exactly as #2176 asks for at the config-schema level -- it just had no direct test coverage, so a regression here would be easy to miss. This PR adds that coverage.

I could not find a helm chart in this repo (grep -rn signingWorker -- *.yaml *.yml turns up nothing), so the "propagate to the helm chart" part of the issue doesn't apply here -- it most likely refers to a different/deployment repo, or the chart has since moved/been removed. Flagging this in case a maintainer wants to redirect that part of the issue elsewhere or close it as already resolved.

Fixes #2176

Test plan

• Verified the exact zod mechanics in isolation (z.preprocess(val => val ?? {}, z.object({ pollInterval: z.number().int().positive().default(5000) })) parsed against {} in this repo's actual zod version) produce { pollInterval: 5000 }, matching the new test's assertion
• Could not run the full vitest suite locally in a fresh worktree -- a sibling package (core-token-standard) requires Daml-generated codegen artifacts not available outside the full dev environment; unrelated to this change

[ayushsingh82]

@rukmini-basu-da whenever you have a chance, would appreciate a look at this one.