Standardize extension handling across all file writing methods

- Enhanced ensureCorrectExtension() method to be more robust:
  - Handle edge cases with null/empty inputs
  - Ensure expectedExt starts with dot
  - Better handling of multiple extensions (e.g., .d.ts, .spec.ts)
- Standardized all file writing methods to use ensureCorrectExtension():
  - writeTypes(), writeEnums(), writeUtilsEnums()
  - writeAstHelpers(), writeWrappedAstHelpers()
  - writeCodeToFile(), writeEnumMaps(), writeRuntimeSchema()
- Consistent extension handling for both TypeScript (.ts) and JSON (.json) files
- Updated test snapshots to reflect corrected file extension behavior
- Prevents double extension bugs (e.g., .ts.ts) across all file generation

Co-Authored-By: Dan Lynch <pyramation@gmail.com>

Author: devin-ai-integration[bot]