refactor(iOS): limit scope of shouldUseCustomBackBarButtonItem flag (#2867)

## Description / Changes

This is continuation of
#2860 that
proposes small refactor around back button logic. This area is very
confusing and in my opinion this is due to flag
`shouldUseCustomBackBarButtonItem` that breaks the flow of other if's
and causes other props to not work. Not we contained this flag in `if
(visible)` which currently is the only case when backButtonItem might be
used after the changes from #2800.

## Test code and steps to reproduce

see `FabricExample/e2e/issuesTests/Test2809.e2e.ts` and
`apps/src/tests/Test2809/index.tsx`

## Checklist
- [ ] Ensured that CI passes

---------

Co-authored-by: Kacper Kafara <[email protected]>

Author: maciekstosio

ios/RNSScreenStackHeaderConfig.mm

@@ -678,20 +678,19 @@ - (void)configureBackItem:(nullable UINavigationItem *)prevItem API_UNAVAILABLE(
 
   const auto isBackTitleBlank = [NSString rnscreens_isBlankOrNull:config.backTitle] == YES;
   NSString *resolvedBackTitle = isBackTitleBlank ? prevItem.title : config.backTitle;
-  RNSUIBarButtonItem *backBarButtonItem = [[RNSUIBarButtonItem alloc] initWithTitle:resolvedBackTitle
-                                                                              style:UIBarButtonItemStylePlain
-                                                                             target:nil
-                                                                             action:nil];
-  [backBarButtonItem setMenuHidden:config.disableBackButtonMenu];
-
-  auto shouldUseCustomBackBarButtonItem = config.disableBackButtonMenu;
-
+  prevItem.backButtonTitle = resolvedBackTitle;
   // This has any effect only in case the `backBarButtonItem` is not set.
   // We apply it before we configure the back item, because it might get overriden.
   prevItem.backButtonDisplayMode = config.backButtonDisplayMode;
-  prevItem.backButtonTitle = resolvedBackTitle;
 
   if (config.isBackTitleVisible) {
+    RNSUIBarButtonItem *backBarButtonItem = [[RNSUIBarButtonItem alloc] initWithTitle:resolvedBackTitle
+                                                                                style:UIBarButtonItemStylePlain
+                                                                               target:nil
+                                                                               action:nil];
+    auto shouldUseCustomBackBarButtonItem = config.disableBackButtonMenu;
+    [backBarButtonItem setMenuHidden:config.disableBackButtonMenu];
+
     if ((config.backTitleFontFamily &&
          // While being used by react-navigation, the `backTitleFontFamily` will
          // be set to "System" by default - which is the system default font.
@@ -715,19 +714,18 @@ - (void)configureBackItem:(nullable UINavigationItem *)prevItem API_UNAVAILABLE(
       }
       [RNSScreenStackHeaderConfig setTitleAttibutes:attrs forButton:backBarButtonItem];
     }
+
+    // Prevent unnecessary assignment of backBarButtonItem if it is not customized,
+    // as assigning one will override the native behavior of automatically shortening
+    // the title to "Back" or hide the back title if there's not enough space.
+    // See: https://github.com/software-mansion/react-native-screens/issues/1589
+    if (shouldUseCustomBackBarButtonItem) {
+      prevItem.backBarButtonItem = backBarButtonItem;
+    }
   } else {
     // back button title should be not visible next to back button,
     // but it should still appear in back menu
     prevItem.backButtonDisplayMode = UINavigationItemBackButtonDisplayModeMinimal;
-    shouldUseCustomBackBarButtonItem = NO;
-  }
-
-  // Prevent unnecessary assignment of backBarButtonItem if it is not customized,
-  // as assigning one will override the native behavior of automatically shortening
-  // the title to "Back" or hide the back title if there's not enough space.
-  // See: https://github.com/software-mansion/react-native-screens/issues/1589
-  if (shouldUseCustomBackBarButtonItem) {
-    prevItem.backBarButtonItem = backBarButtonItem;
   }
 #endif
 }