feat: Add comprehensive improvements - async optimization, caching, and error handling

- Add async optimization using futures for concurrent file processing
- Implement intelligent caching system with SHA256 file change detection
- Add comprehensive custom error handling for Gemini API responses
- Fix critical error handling issues throughout the codebase
- Replace fragile JSON parsing with proper struct-based deserialization
- Add automatic retry logic for rate limits and network issues
- Improve user experience with detailed error messages and progress feedback
- Add cache persistence and automatic cleanup of old entries
- Optimize performance for batch processing scenarios

src/lib.rs

@@ -1,2 +1,4 @@
+pub mod cache;
 pub mod files;
 pub mod gemini;
+pub mod gemini_errors;