test: Add comprehensive test suite for basic functionality

- Add 31 tests across files, config, and cache modules
- Test file operations (FileBatch, is_text_file, read_file_sample)
- Test configuration (serialization, API key validation, path expansion)
- Test caching (response retrieval, file change detection, eviction)
- Add tempfile dev dependency for test fixtures
- All tests passing with 100% success rate

Cargo.toml

@@ -16,3 +16,6 @@ sha2 = "0.10.8"
 thiserror = "2.0.11"
 tokio = { version = "1.48.0", features = ["full"] }
 toml = "0.8.19"
+
+[dev-dependencies]
+tempfile = "3.15"