Add edge case and success tests for compress_zlib_into (#421)

- Added `test_compress_zlib_into_success` to verify successful compression into a provided buffer.
- Added `test_compress_zlib_into_insufficient_space` to verify correct error handling when the output buffer is too small.
- Mirrored existing gzip compression testing patterns for consistency.
- Verified decompression of zlib-compressed data in the success case.

Author: 404Setup

tests/unit_tests.rs

@@ -182,6 +182,25 @@ fn test_compress_gzip_into_success() {
     assert_eq!(data.to_vec(), decompressed);
 }
 
+#[test]
+fn test_compress_zlib_into_success() {
+    let mut compressor = Compressor::new(6).unwrap();
+    let mut decompressor = Decompressor::new();
+    let data = b"Hello world! This is a test string for zlib compression into buffer.";
+
+    let bound = compressor.zlib_compress_bound(data.len());
+    let mut output = vec![0u8; bound];
+
+    let size = compressor.compress_zlib_into(data, &mut output).unwrap();
+    assert!(size > 0);
+    assert!(size <= bound);
+
+    let decompressed = decompressor
+        .decompress_zlib(&output[..size], data.len())
+        .unwrap();
+    assert_eq!(data.to_vec(), decompressed);
+}
+
 #[test]
 fn test_compress_gzip_into_insufficient_space() {
     let mut compressor = Compressor::new(6).unwrap();
@@ -192,6 +211,16 @@ fn test_compress_gzip_into_insufficient_space() {
     assert!(result.is_err());
 }
 
+#[test]
+fn test_compress_zlib_into_insufficient_space() {
+    let mut compressor = Compressor::new(6).unwrap();
+    let data = b"Hello world! This is a test string for zlib compression.";
+
+    let mut output = vec![0u8; 10];
+    let result = compressor.compress_zlib_into(data, &mut output);
+    assert!(result.is_err());
+}
+
 #[test]
 fn test_new_compressor_invalid_level() {
     let res = Compressor::new(-1);