fix(offline): reject truncated downloads when Content-Length is known

Fail closed on short bodies and empty payloads so partial files never
become complete cache entries.

crates/qbz-offline-cache/src/downloader.rs

@@ -113,6 +113,27 @@ impl StreamFetcher {
         Err(last_error)
     }
 
+
+/// Validate downloaded byte count against optional Content-Length.
+/// Fail closed when length is known and mismatched, or when zero bytes.
+pub(crate) fn validate_download_size(
+    track_id: u64,
+    cached: u64,
+    content_length: Option<u64>,
+) -> Result<(), String> {
+    if let Some(expected) = content_length {
+        if cached != expected {
+            return Err(format!(
+                "Truncated download for track {track_id}: got {cached} bytes, Content-Length was {expected}"
+            ));
+        }
+    }
+    if cached == 0 {
+        return Err(format!("Empty download for track {track_id}"));
+    }
+    Ok(())
+}
+
     /// Single download attempt: stream response body to a temp file.
     async fn try_download(
         &self,
@@ -214,6 +235,8 @@ impl StreamFetcher {
             .map_err(|e| format!("Failed to flush file: {}", e))?;
         drop(file);
 
+        validate_download_size(track_id, cached, total_size)?;
+
         Ok(cached)
     }
 
@@ -707,3 +730,31 @@ pub fn spawn_track_cache_download(
         }
     });
 }
+
+#[cfg(test)]
+mod download_size_tests {
+    use super::validate_download_size;
+
+    #[test]
+    fn accepts_matching_length() {
+        assert!(validate_download_size(1, 100, Some(100)).is_ok());
+    }
+
+    #[test]
+    fn rejects_truncated() {
+        let err = validate_download_size(7, 50, Some(100)).unwrap_err();
+        assert!(err.contains("Truncated"));
+        assert!(err.contains("50"));
+        assert!(err.contains("100"));
+    }
+
+    #[test]
+    fn rejects_empty_even_without_length() {
+        assert!(validate_download_size(2, 0, None).is_err());
+    }
+
+    #[test]
+    fn accepts_nonzero_unknown_length() {
+        assert!(validate_download_size(3, 42, None).is_ok());
+    }
+}