fix properly interpolate variables in generate.rs error messages

[anaslimem]

Summary

Fixes incorrect error message formatting in src/bin/generate.rs where file write failures would display literal placeholder text instead of actual values.

Changes

Fixed three locations where .expect() was used with format string placeholders that were never interpolated:

• Line 69: schema_file - Fixed error when writing schema JSON
• Line 95: meta_file - Fixed error when writing metadata JSON
• Line 107: doc_file - Fixed error when writing markdown documentation

Before vs After

Scenario	Before (broken)	After (fixed)
Write fails	Failed to write {schema_file}	Failed to write schema.json: Permission denied

Testing

• Verified the fix compiles without clippy warnings
• Created a temporary test that triggered the error path to confirm proper variable interpolation