feat: add explicit terminal state to search snapshot

[mountiny]

Problem

Whether a search snapshot has finished loading is inferred from a mix of fields (data, isLoading, type/status), which breaks when a request resolves with HTTP 200 but writes no data at all — nothing marks the snapshot as done, leaving the page stuck in a loading state. Search requests are read commands that never enter the offline queue, so the snapshot must own its own lifecycle.

Solution

Add an explicit state field (loading / loaded / error) to the search snapshot. getOnyxLoadingData writes loading optimistically on request start, a new successData branch writes loaded on any successful resolve (even without snapshot data), and failureData writes error. finallyData intentionally leaves state alone to avoid overwriting the error result. Network-level rejections apply failureData in a .catch() handler. This PR is write-side only; UI gates will migrate in a follow-up.

PR

#96043

Issue Owner

Current Issue Owner: @BartekObudzinski