Fix Google Drive root browse returning an empty list

[jzongker]

Browsing Google Drive returned nothing on production (ChurchAppsSupport #944).

Root cause

GoogleDriveProvider.listFiles sent includeItemsFromAllDrives=true alongside a 'root' in parents query. root is an alias that only resolves inside the user's own corpus, so pairing it with an all-drives search made Google reject the request. ApiHelper.apiRequest turned the non-OK response into null, listFiles broke out of its pagination loop with no entries, and browse returned [] — an empty folder rather than an error.

Changes

• listFiles: keep supportsAllDrives=true, drop includeItemsFromAllDrives, and set corpora=user. Ordering (folder,name_natural) is unchanged.
• ApiHelper.apiRequest: on a non-OK response, log the response body truncated to 500 chars so the provider's actual reason shows up in logs instead of just the status code.
• Tests: assert the root browse sends supportsAllDrives=true + corpora=user and no includeItemsFromAllDrives, and that a failed fetch yields []. Existing audio/octet-stream coverage is untouched.

All 34 content-providers tests pass.