Skills/Python: harden script edge cases and add regression tests (#24277)

* Skill creator: skip self-including .skill output

* Skill creator tests: cover output-dir-inside-skill case

* Skill validator: parse frontmatter robustly across newlines

* Skill validator tests: add CRLF and malformed frontmatter coverage

* Model usage: require positive --days value

* Model usage tests: cover --days validation and filtering

* Nano banana: close input image handles after loading

* Skill validator: keep type hints compatible with older python

* Changelog: credit @vincentkoc for Python skills hardening

skills/model-usage/scripts/model_usage.py

@@ -17,6 +17,16 @@ from datetime import date, datetime, timedelta
 from typing import Any, Dict, Iterable, List, Optional, Tuple
 
 
+def positive_int(value: str) -> int:
+    try:
+        parsed = int(value)
+    except ValueError as exc:
+        raise argparse.ArgumentTypeError("must be an integer") from exc
+    if parsed < 1:
+        raise argparse.ArgumentTypeError("must be >= 1")
+    return parsed
+
+
 def eprint(msg: str) -> None:
     print(msg, file=sys.stderr)
 
@@ -239,7 +249,7 @@ def main() -> int:
     parser.add_argument("--mode", choices=["current", "all"], default="current")
     parser.add_argument("--model", help="Explicit model name to report instead of auto-current.")
     parser.add_argument("--input", help="Path to codexbar cost JSON (or '-' for stdin).")
-    parser.add_argument("--days", type=int, help="Limit to last N days (based on daily rows).")
+    parser.add_argument("--days", type=positive_int, help="Limit to last N days (based on daily rows).")
     parser.add_argument("--format", choices=["text", "json"], default="text")
     parser.add_argument("--pretty", action="store_true", help="Pretty-print JSON output.")