Fix GOG cloud save fetch failure handling

[kiequoo]

Summary

This PR fixes two bugs in GOG cloud save sync and one related support issue in save-location resolution.

Before this change, getCloudFiles() treated request or parse failures the same as a valid empty cloud result. That could make sync logic believe the cloud was empty and incorrectly choose an upload path. It also parsed GOG last_modified timestamps with Instant.parse(...), which does not handle the offset-based format GOG returns, so remote timestamps could be dropped during sync decisions.

It also stops fabricating a default GOG cloud-save path when the remote config API returns no save locations. In that case, the game should be treated as not supporting cloud saves rather than showing a misleading fallback path with no usable credentials.

Changes

• Return null from GOG cloud file listing on HTTP or parse failure instead of emptyList()
• Abort sync when cloud file listing fails instead of treating the failure as cloud is empty
• Parse GOG cloud timestamps with OffsetDateTime.parse(...).toInstant()
• Stop falling back to a fake default cloud-save location when the API reports no save locations
• Add regression tests covering offset timestamp parsing and failure-vs-empty cloud responses

Testing

• ./gradlew :app:compileDebugKotlin
• ./gradlew :app:testDebugUnitTest --tests app.gamenative.service.gog.GOGCloudSavesManagerTest