fix: stabilize iOS CI toolchain

[mym0404]

Overview

• Pin Ruby 3.1.2 in the iOS CI job so CocoaPods does not use the runner's incompatible Ruby 3.4 default.
• Select Xcode 26.2, which matches an installed iOS Simulator runtime on the current runner image.
• Verify the matching Simulator runtime before checkout and download it when the runner image does not provide it.

Verification

• Parse .github/workflows/ci.yml with Ruby YAML
• Validate the Simulator runtime fallback with bash -n
• Run git diff --check
• Confirm Ruby setup and CocoaPods installation pass in GitHub Actions
• Confirm the updated GitHub Actions iOS build passes