[codex] Include iOS release dart defines

[jjoonleo]

Summary

• Include Dart-Defines.xcconfig in iOS release builds so plist substitutions match debug behavior.
• Harden the Dart define extraction script and fail release/archive builds when GOOGLE_RESERVED_CLIENT_ID_IOS is missing.
• Add a release-only Info.plist validation phase and document the required iOS release define.

Why

Release builds previously did not include the generated Dart define xcconfig, while Info.plist depends on $(GOOGLE_RESERVED_CLIENT_ID_IOS) for the Google Sign-In redirect scheme. That could produce release/archive apps with a missing or unresolved callback URL scheme.

Fixes #387

Validation

• bash -n ios/scripts/extract_dart_defines.sh ios/scripts/validate_release_info_plist.sh
• git diff --check
• Verified release define extraction succeeds with GOOGLE_RESERVED_CLIENT_ID_IOS.
• Verified missing release define fails with the intended error.
• Verified release Info.plist validation passes/fails against sample plists.
• xcodebuild -list -project ios/Runner.xcodeproj
• Attempted release simulator build after flutter pub get and pod install; it reached native pod compilation and then stopped with No space left on device.

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