fix: open custom unified search file results

[Dennis-Otto]

Summary

• Resolve custom unified search entries with fileId or path attributes to native Nextcloud file metadata.
• Keep the existing URL fallback for results that do not refer to a Nextcloud file.
• Preserve the existing behaviour of the built-in files and fulltextsearch providers.

Reasoning

Nextcloud iOS currently creates temporary URL metadata for every result from a custom unified search provider, even when the entry exposes the standard fileId and path attributes. Selecting that temporary item exits silently because its random ocId is not present in Realm.

Resolving those attributes before creating the URL fallback makes file-backed results use the regular native file metadata and viewer. It also gives them their actual file type and preview instead of a generic URL icon.

Closes #4274.

Testing

• Reproduced with Nextcloud iOS 34.1.4 against an isolated Nextcloud 33.0.8 instance and a minimal custom search provider.
• Confirmed that the provider response contains a readable file ID and user-relative path.
• Ran git diff --check.
• Parsed the changed Swift source with xcrun swiftc -frontend -parse.
• A full Xcode build was not available locally; the project CI will perform the supported simulator build and test suite.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI