Fix pr comments

scripts/codegen/update_codegen_stamp.py

@@ -27,6 +27,9 @@ def compute_combined_hash(input_files: list[str]) -> str:
     """
     parts = []
     for filepath in input_files:
+        if not Path(filepath).exists():
+            print(f"Error: input file not found: {filepath}", file=sys.stderr)
+            sys.exit(1)
         file_hash = hashlib.sha256(Path(filepath).read_bytes()).hexdigest()
         parts.append(file_hash)
 
@@ -39,7 +42,7 @@ def read_stamp_hash(stamp_file: str) -> str:
     path = Path(stamp_file)
     if not path.exists():
         return ""
-    for line in path.read_text().splitlines():
+    for line in path.read_text(encoding="utf-8").splitlines():
         if line.startswith("COMBINED_HASH="):
             return line.split("=", 1)[1]
     return ""
@@ -67,7 +70,7 @@ def main():
             sys.exit(1)
 
     # --update
-    with open(stamp_file, "w") as fp:
+    with open(stamp_file, "w", encoding="utf-8") as fp:
         fp.write(
             "# Auto-generated by protocol autogen - do not edit manually.\n"
             "# This file tracks input hashes to avoid unnecessary code regeneration.\n"