fix: crash in topIndexPath - WPB-23912

[netbe]

WPB-23912 [iOS] crash on topIndexPath(for:) ConversationTableViewDataSource.swift:571

Issue

The app crashed when scrolling to a specific message (e.g. via push notification or search result) in a long conversation while background sync was active.

topIndexPath(for:) called tableView.numberOfRows(inSection:) with a section index derived from the fetch controller. Because calculateSections snapshots allMessages before dispatching to a background thread, the
fetch controller can gain new objects (via NSFetchedResultsControllerDelegate) by the time the result is applied and reloadData() is called. This made the fetch controller index exceed the number of sections the
table view knew about, triggering UIKit's internal assertion.

The fix changes index(of:) to look up the message by nonce in currentSections instead of the fetch controller. Since currentSections is always the exact snapshot the table view was last reloaded with, the
returned index is guaranteed to be in bounds.

Testing

1. Open a conversation with significantly more than 30 messages.
2. While the conversation is open and receiving new messages, trigger a scroll to an old message — either by tapping a push notification or using message search and jumping to a result.
3. Verify the app does not crash and scrolls to the correct message.

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Checklist

• Title contains a reference JIRA issue number like [WPB-XXX].
• Description is filled and free of optional paragraphs.
• Adds/updates automated tests.

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UI accessibility checklist

If your PR includes UI changes, please utilize this checklist:

• Make sure you use the API for UI elements that support large fonts.
• All colors are taken from WireDesign.ColorTheme or constructed using WireDesign.BaseColorPalette.
• New UI elements have Accessibility strings for VoiceOver.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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