fix: apply headerTintColor to native stack bar button items on iOS 26

[Priyanshu0007]

Closes #12932

Motivation

On iOS 26 (Liquid Glass), UIKit isolates UIBarButtonItems inside shared capsule toolbars so they do not inherit navigationBar.tintColor. Instead, iOS 26 requires tintColor to be set explicitly on each individual UIBarButtonItem.

In @react-navigation/native-stack, processBarButtonItems was not forwarding tintColor to headerLeftBarButtonItems and headerRightBarButtonItems unless explicitly set per-item in screen options. Because of this, custom header items and buttons remained in the default system label color (black) on iOS 26 when headerTintColor was provided.

This PR updates processBarButtonItems in useHeaderConfigProps.tsx to automatically default tintColor to the screen's resolved tintColor (from headerTintColor), ensuring consistent header item tinting across iOS 26, older iOS versions, and Android.

Test plan

1. Set headerTintColor on a native stack screen (e.g. headerTintColor: "#198a00ff").
2. Defined unstable_headerLeftItems and unstable_headerRightItems with buttons and menus.
3. Verified on iOS 26.5 (iPhone 17 Pro), iOS 16.4 (iPhone 14 Pro), and Android (Pixel 10):
   • Header title, left items, right items, and SF symbols are tinted consistently without requiring manual per-item overrides.
4. Ran all verification commands:
   • pnpm typecheck (Passed)
   • pnpm lint (Passed)
   • pnpm test (All 81 test suites passed)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.92%. Comparing base (bce29d6) to head (0d79761).
⚠️ Report is 310 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13228      +/-   ##
==========================================
+ Coverage   72.24%   77.92%   +5.67%     
==========================================
  Files         238      264      +26     
  Lines        8391    10722    +2331     
  Branches     3480     4442     +962     
==========================================
+ Hits         6062     8355    +2293     
- Misses       2274     2304      +30     
- Partials       55       63       +8     

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