Fix Workshop-only updates without a Minecraft version

[QIU-Guanzong]

Summary

This is a derived, competing PR based on #291. It preserves that PR's Workshop implementation and adds a narrow regression fix for its current head:

• defer Minecraft-version discovery until an included mod actually uses the Modrinth/CurseForge lookup path;
• retain the configured server-updater.version fallback before probing the server executable;
• add a Workshop-only regression test where no Minecraft version is configured and the server executable is not a JAR.

The current #291 code eagerly calls Server.getMCVersion() before it reaches its Steam Workshop branch. That makes a DayZ Workshop-only update check fail when the server executable cannot provide a Minecraft version. The new test fails against #291's current head and passes with this branch.

An earlier public issue comment describes the same condition. This PR does not claim exclusive discovery; it provides a reviewable derived branch with the focused regression test for the competing-PR process.

Validation

/Users/c-gavin.yau/Documents/Codex/2026-09-21/github-bounty-pipeline/work/tool-cache/apache-maven-3.9.16/bin/mvn -B -Dstyle.color=never -Dtest=SteamWorkshopModTest test

Result: 5 tests run, 0 failures, 0 errors.

git diff --check also passes.

The test uses synthetic files and a fake SteamCMD boundary. No live DayZ server or Steam operation was run, and no full test-suite result is claimed.

AI assistance and scope

This contribution was developed with substantial AI assistance. The automated validation above is reported exactly as run; this PR does not claim human validation. Please review the implementation and test before considering it for merge.

This PR does not claim bounty eligibility, payment, or any payment identity details.

[Osiris-Team]

Original base PR was merged.