Cap CI Xcode version at 26.3 to fix iOS build

[CassioMG]

What

Cap the auto-selected Xcode version at 26.3 in the CI setup script.

Why

Xcode 26.4 (recently launched on March 24, 2026) ships a stricter Clang that breaks the fmt CocoaPod with consteval errors:

ios/Pods/fmt/include/fmt/format-inl.h:59:24: error: call to consteval function
'fmt::basic_format_string<...>::basic_format_string<FMT_COMPILE_STRING, 0>'
is not a constant expression

The setup-xcode-latest-stable script auto-selects the highest non-beta Xcode on the runner. Since 26.4 is now available on CI runners, all iOS builds fail.

Changes

Added a MAX_XCODE_VERSION variable at the top of scripts/setup-xcode-latest-stable that caps selection at 26.3. Versions above the cap are excluded before the sort, so CI will pick 26.3.x as the latest allowed version.

To bump later, just change the single variable at the top of the script once the fmt pod (or its consumer) is updated.

Known limitations

N/A — this is a temporary cap until pod compatibility is resolved.

Checklist

PR structure

• This PR does not mix refactoring changes with feature changes.
• This PR has reasonably narrow scope.
• This PR includes relevant before and after screenshots/videos highlighting these changes.
• I took the time to review my own PR.

Testing

• Version comparison logic tested locally (26.4 excluded, 26.3 included, 26.2 included).

Release

• This is not a breaking change.

[Copilot]

Pull request overview

This PR updates the CI Xcode selection helper to prevent iOS builds from picking a newer Xcode that breaks CocoaPods compilation (notably fmt) by capping the auto-selected version.

Changes:

• Introduced a MAX_XCODE_VERSION constant to cap selection to a maximum major.minor Xcode version.
• Added exclusion logic to skip any stable Xcode installations whose major.minor version is above the cap, before sorting to pick the latest allowed version.

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