fix(context): only chmod newly created dirs in SaveUploadedFile

Fixes #4622 - SaveUploadedFile no longer attempts to modify
permissions on existing directories, which breaks saving to /tmp
and other system directories.

- Check if directory exists before calling os.MkdirAll
- Only call os.Chmod if directory was newly created
- Preserves backward compatibility for newly created dirs
- Allows saving files to existing system directories

This regression was introduced in v1.12.0 when chmod support
was added, but was incorrectly applied to all directories.

BREAKING CHANGE REVERTED: Restores v1.10.1 behavior for existing dirs

Co-authored-by: Nghia Nguyen <nghiack7@users.noreply.github.com>

context.go

@@ -728,11 +728,17 @@ func (c *Context) SaveUploadedFile(file *multipart.FileHeader, dst string, perm
 		mode = perm[0]
 	}
 	dir := filepath.Dir(dst)
+	// Check if directory exists to only chmod newly created directories
+	_, statErr := os.Stat(dir)
+	dirExists := !os.IsNotExist(statErr)
 	if err = os.MkdirAll(dir, mode); err != nil {
 		return err
 	}
-	if err = os.Chmod(dir, mode); err != nil {
-		return err
+	// Only chmod if directory was newly created (fixes #4622)
+	if !dirExists {
+		if err = os.Chmod(dir, mode); err != nil {
+			return err
+		}
 	}
 
 	out, err := os.Create(dst)