refactor: Separate tests into dedicated files to reduce source file sizes

- Extract all tests from cache.rs, files.rs, config.rs to separate test files
- Create cache_tests.rs, files_tests.rs, config_tests.rs
- Reduce cache.rs: 421 -> 220 lines (-191 lines)
- Reduce files.rs: 390 -> 264 lines (-126 lines)
- Reduce config.rs: 354 -> 275 lines (-79 lines)
- Total reduction: 396 lines from main source files
- All 31 tests still passing

src/files_tests.rs

@@ -0,0 +1,126 @@
+use crate::files::*;
+use std::fs::{self, File};
+use std::io::Write;
+
+#[test]
+fn test_is_text_file_with_text_extensions() {
+    assert!(is_text_file(Path::new("test.txt")));
+    assert!(is_text_file(Path::new("test.rs")));
+    assert!(is_text_file(Path::new("test.py")));
+    assert!(is_text_file(Path::new("test.md")));
+    assert!(is_text_file(Path::new("test.json")));
+}
+
+#[test]
+fn test_is_text_file_with_binary_extensions() {
+    assert!(!is_text_file(Path::new("test.exe")));
+    assert!(!is_text_file(Path::new("test.bin")));
+    assert!(!is_text_file(Path::new("test.jpg")));
+    assert!(!is_text_file(Path::new("test.pdf")));
+}
+
+#[test]
+fn test_is_text_file_case_insensitive() {
+    assert!(is_text_file(Path::new("test.TXT")));
+    assert!(is_text_file(Path::new("test.RS")));
+    assert!(is_text_file(Path::new("test.Py")));
+}
+
+#[test]
+fn test_file_batch_from_path() {
+    let temp_dir = tempfile::tempdir().unwrap();
+    let dir_path = temp_dir.path();
+
+    File::create(dir_path.join("file1.txt")).unwrap();
+    File::create(dir_path.join("file2.rs")).unwrap();
+    fs::create_dir(dir_path.join("subdir")).unwrap();
+
+    let batch = FileBatch::from_path(dir_path.to_path_buf());
+    assert_eq!(batch.count(), 2);
+    assert!(batch.filenames.contains(&"file1.txt".to_string()));
+    assert!(batch.filenames.contains(&"file2.rs".to_string()));
+}
+
+#[test]
+fn test_file_batch_from_path_nonexistent() {
+    let batch = FileBatch::from_path(PathBuf::from("/nonexistent/path"));
+    assert_eq!(batch.count(), 0);
+}
+
+#[test]
+fn test_read_file_sample() {
+    let temp_dir = tempfile::tempdir().unwrap();
+    let file_path = temp_dir.path().join("test.txt");
+
+    let mut file = File::create(&file_path).unwrap();
+    file.write_all(b"Hello, World!").unwrap();
+
+    let content = read_file_sample(&file_path, 1000);
+    assert_eq!(content, Some("Hello, World!".to_string()));
+}
+
+#[test]
+fn test_read_file_sample_with_limit() {
+    let temp_dir = tempfile::tempdir().unwrap();
+    let file_path = temp_dir.path().join("test.txt");
+
+    let mut file = File::create(&file_path).unwrap();
+    file.write_all(b"Hello, World! This is a long text.").unwrap();
+
+    let content = read_file_sample(&file_path, 5);
+    assert_eq!(content, Some("Hello".to_string()));
+}
+
+#[test]
+fn test_read_file_sample_binary_file() {
+    let temp_dir = tempfile::tempdir().unwrap();
+    let file_path = temp_dir.path().join("test.bin");
+
+    let mut file = File::create(&file_path).unwrap();
+    file.write_all(&[0x00, 0xFF, 0x80, 0x90]).unwrap();
+
+    let content = read_file_sample(&file_path, 1000);
+    assert_eq!(content, None);
+}
+
+#[test]
+fn test_read_file_sample_nonexistent() {
+    let content = read_file_sample(Path::new("/nonexistent/file.txt"), 1000);
+    assert_eq!(content, None);
+}
+
+#[test]
+fn test_organization_plan_serialization() {
+    let plan = OrganizationPlan {
+        files: vec![
+            FileCategory {
+                filename: "test.txt".to_string(),
+                category: "Documents".to_string(),
+                sub_category: "Text".to_string(),
+            },
+        ],
+    };
+
+    let json = serde_json::to_string(&plan).unwrap();
+    assert!(json.contains("test.txt"));
+    assert!(json.contains("Documents"));
+
+    let deserialized: OrganizationPlan = serde_json::from_str(&json).unwrap();
+    assert_eq!(deserialized.files[0].filename, "test.txt");
+}
+
+#[test]
+fn test_file_category_serialization() {
+    let fc = FileCategory {
+        filename: "file.rs".to_string(),
+        category: "Code".to_string(),
+        sub_category: "Rust".to_string(),
+    };
+
+    let json = serde_json::to_string(&fc).unwrap();
+    let deserialized: FileCategory = serde_json::from_str(&json).unwrap();
+
+    assert_eq!(fc.filename, deserialized.filename);
+    assert_eq!(fc.category, deserialized.category);
+    assert_eq!(fc.sub_category, deserialized.sub_category);
+}