fix(gateway): address review feedback from 普渡法師

1. media.rs: use DimensionError instead of InsufficientMemory for GIF
   size limit (correct semantic — size cap, not OOM)
2. line.rs: derive audio filename extension from Content-Type instead
   of hardcoding .m4a
3. telegram.rs: reject non-UTF-8 documents with return None instead of
   silently corrupting via from_utf8_lossy

Author: chaodu-agent

gateway/src/adapters/line.rs

@@ -344,9 +344,15 @@ async fn download_line_media(
             }
         }
     } else {
-        // For audio, we don't process, just send as is.
-        // LINE audio is usually m4a.
-        (bytes.to_vec(), content_type, format!("{}.m4a", message_id))
+        // Derive extension from Content-Type
+        let ext = if content_type.contains("mpeg") || content_type.contains("mp3") {
+            "mp3"
+        } else if content_type.contains("ogg") {
+            "ogg"
+        } else {
+            "m4a"
+        };
+        (bytes.to_vec(), content_type, format!("{}.{}", message_id, ext))
     };
 
     use base64::Engine;

gateway/src/adapters/telegram.rs

@@ -491,7 +491,13 @@ async fn download_telegram_document(
         return None;
     }
 
-    let text = String::from_utf8_lossy(&bytes);
+    let text = match String::from_utf8(bytes.to_vec()) {
+        Ok(t) => t,
+        Err(_) => {
+            warn!(file_id, file_name, "Telegram document is not valid UTF-8, skipping");
+            return None;
+        }
+    };
     use base64::Engine;
     let data = base64::engine::general_purpose::STANDARD.encode(text.as_bytes());
     info!(file_id, file_name, size = bytes.len(), "Telegram document download successful");

gateway/src/media.rs

@@ -16,7 +16,7 @@ pub fn resize_and_compress(raw: &[u8]) -> Result<(Vec<u8>, String), image::Image
     if format == Some(image::ImageFormat::Gif) {
         if raw.len() > GIF_MAX_SIZE {
             return Err(image::ImageError::Limits(
-                image::error::LimitError::from_kind(image::error::LimitErrorKind::InsufficientMemory),
+                image::error::LimitError::from_kind(image::error::LimitErrorKind::DimensionError),
             ));
         }
         return Ok((raw.to_vec(), "image/gif".to_string()));