fix: pull to refresh mechanism - WPB-21415 🍒

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This PR was automatically cherry-picked based on the following PR:

• fix: pull to refresh mechanism - WPB-21415 #3820

Original PR description:

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WPB-21415 [iOS] Pull to refresh hides the search bar and whole page shows blank for a fraction of second

Issue

This PR fixes a couple of issues found on the files views:

• pull to refresh mechanism which had a weird UI behavior because we were relying on our own loading mechanism instead of the built-in one.
• files not being updated properly because we were passing an empty searchText input to the API instead of nil when there's no active search.
• when showing up for the first time, files views seem to be loading twice causing a small UI glitch. issue is that the search binding is initially triggered twice consecutively whereas it should not be.

Testing

• when pulling to refresh whether in the Files tab or in the convo view, current items should still be visible, no blank screen and the search bar still shows up in the files tabs. then loading indicator shows up and refreshes the items properly

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.

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Test Results

203 tests   203 ✅  5m 47s ⏱️
 41 suites    0 💤
  1 files      0 ❌

Results for commit 3006f90.