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.lock

@@ -830,6 +830,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sha2",
+ "tempfile",
  "thiserror 2.0.17",
  "tokio",
  "toml",