Fix string indexing and eliminate warnings for moon check

[peter-jerry-ye]

Summary

Fixed type conversion issues and eliminated warnings throughout the codebase to ensure moon check works properly. The main change addresses the string indexing behavior where string[i] now returns UInt16 instead of Int.

Changes

• String indexing fix: Updated string indexing operations to handle the new UInt16 return type with proper type conversion
• Warning elimination: Made minimal changes across 23 files to resolve compiler warnings
• Type consistency: Ensured all generated .mbti files and source files use consistent types

Implementation Details

The primary issue was that string indexing behavior changed from returning Int to UInt16. This required:

• Adding type conversion where string indexing results are used
• Updating type annotations in generated interface files
• Ensuring type consistency across the entire codebase

All changes were made with minimal modifications to preserve existing functionality while resolving the type system warnings.

Verification

The changes ensure that:

• moon check passes without warnings
• moon test should work (if applicable)
• No breaking changes to the public API
• Type safety is maintained throughout the codebase