fix: use actual parsed string in literal size error message

The error format string was logging s[i+1:j] (which includes '+' from
LITERAL+ syntax) but Atoi receives s[i+1:sizeEnd] (numeric only).
Log the same string that was actually passed to Atoi.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: BrianLeishman

parse.go

@@ -298,7 +298,7 @@ func findFetchContentEnd(s string, fetchContentStart int) (int, error) {
 				}
 				size, err := strconv.Atoi(s[i+1 : sizeEnd])
 				if err != nil {
-					return 0, fmt.Errorf("parse literal size %q: %w", s[i+1:j], err)
+					return 0, fmt.Errorf("parse literal size %q: %w", s[i+1:sizeEnd], err)
 				}
 
 				i = j + 1