Stop PCGamingWiki failures from poisoning the feature cache

[dguay]

What to build

Treat PCGamingWiki API failures as failures even when the server responds with HTTP 200. A failed refresh must not look like a legitimate undocumented game or overwrite cached PC Features with empty values. Users without cached data should see the existing refresh-error state. Users with valid cached data should keep those values when a refresh fails.

Acceptance criteria

• A MediaWiki response containing an error envelope, including permissiondenied, rejects the feature request instead of becoming a null result.
• Failed primary lookups do not insert an empty cache row, clear a documented row, or advance its freshness timestamp.
• A failed refresh preserves existing valid cached features.
• A failed lookup with no cache reaches the user-visible PC Features error state.
• Tests reproduce the HTTP 200 plus permissiondenied response and protect the cache behavior at the real lookup boundary.
• A genuine successful no-match response can still be cached as an undocumented game.

Blocked by

None. This issue can start immediately.