fix(agent): close base64 encoder on io.Copy error path

Previously when io.Copy to the base64 encoder failed, encoder.Close()
was skipped. This left the encoder's internal buffer unflushed.
Now always call Close() and handle both copy and close errors
explicitly.

pkg/agent/agent_media.go

@@ -172,14 +172,22 @@ func encodeImageToDataURL(localPath, mime string, info os.FileInfo, maxSize int)
 	buf.WriteString(prefix)
 
 	encoder := base64.NewEncoder(base64.StdEncoding, &buf)
-	if _, err := io.Copy(encoder, f); err != nil {
+	_, copyErr := io.Copy(encoder, f)
+	closeErr := encoder.Close()
+	if copyErr != nil {
 		logger.WarnCF("agent", "Failed to encode media file", map[string]any{
 			"path":  localPath,
-			"error": err.Error(),
+			"error": copyErr.Error(),
+		})
+		return ""
+	}
+	if closeErr != nil {
+		logger.WarnCF("agent", "Failed to close base64 encoder", map[string]any{
+			"path":  localPath,
+			"error": closeErr.Error(),
 		})
 		return ""
 	}
-	encoder.Close()
 
 	return buf.String()
 }