Fix iOS Entry password text loss when toggling IsPassword while focused

[Copilot]

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Description

This PR fixes an iOS-specific issue where an Entry with IsPassword=true would lose previously entered text when toggling password visibility while the field is focused.

Problem

On iOS, when following these steps:

1. Enter text in a password Entry field (e.g., "password123")
2. Toggle IsPassword = false to show the text
3. Toggle IsPassword = true to hide the text again
4. Type additional text (e.g., "456")

The previously entered text would be lost, resulting in only "456" instead of the expected "password123456".

Root Cause

The issue was in the UpdateIsPassword method in TextFieldExtensions.cs for iOS. When toggling back to password mode while the field is focused, the existing workaround code:

1. Temporarily disables the text field (textField.Enabled = false)
2. Sets secure text entry (textField.SecureTextEntry = true)
3. Re-enables the field (textField.Enabled = entry.IsEnabled)
4. Restores focus (textField.BecomeFirstResponder())

This disable/enable cycle on iOS was inadvertently clearing the text field content.

Solution

Added minimal text preservation logic that:

• Stores the current text before the disable/enable cycle
• Checks if text was lost after re-enabling
• Restores the text only if it was cleared

// Store the current text before disabling to prevent text loss
var currentText = textField.Text;
textField.Enabled = false;
textField.SecureTextEntry = true;
textField.Enabled = entry.IsEnabled;
// Restore the text after re-enabling if it was cleared
if (textField.Text != currentText)
    textField.Text = currentText;
textField.BecomeFirstResponder();

Testing

• Added unit test PasswordTogglingPreservesTextWhenFocused in EntryHandlerTests.iOS.cs
• Created manual test case Issue30085.cs for validation
• Verified the fix compiles for all target frameworks
• No breaking changes or impact on other platforms

Impact

• ✅ Fixes text loss when toggling password visibility on iOS
• ✅ Maintains all existing behavior and workarounds
• ✅ Minimal, surgical change affecting only the problematic code path
• ✅ No impact on Android, Windows, or other platforms
• ✅ Backward compatible

Fixes #30085.

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[AlleSchonWeg]

It would be nice to also keep the cursor position when toggling.

[sheiksyedm]

Closed as favour of #30572