Detect YAML inputs by extension and report a missing file cleanly

Two defects in YAML handling:

- findYamls classified any token *containing* ".yaml"/".yml" as an input
  file (substring match). An option value like -prefix=run.yaml.bak was
  misclassified, then skipped from option parsing (line 122) and silently
  dropped, and fed to YAML::LoadFile. Match a trailing extension instead.

- setOptionsValuesFromYamlFile caught only YAML::ParserException.
  YAML::LoadFile throws YAML::BadFile (a sibling, both derive from
  YAML::Exception) for a missing/unreadable path, so a mistyped *.yaml
  filename escaped the catch and called std::terminate. Add a BadFile
  catch with a clean "cannot open" message.

Verified: "gemc nonexistent.yaml" now prints "Cannot open yaml file ...
Check the path and spelling." and exits cleanly instead of terminating
with an uncaught exception (Geant4 11.4.1 dev container).

Fixes #125

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: zhaozhiwen

gemc/goptions/goptions.cc

@@ -284,12 +284,15 @@ void GOptions::printOptionOrSwitchHelp(const std::string& tag) const {
 // Private method: see header. Kept undocumented here to avoid duplicate param docs.
 vector<string> GOptions::findYamls(int argc, char* argv[]) {
 	vector<string> yaml_files;
+	auto ends_with = [](const string& s, const string& suffix) {
+		return s.size() >= suffix.size() &&
+		       s.compare(s.size() - suffix.size(), suffix.size(), suffix) == 0;
+	};
 	for (int i = 1; i < argc; i++) {
 		string arg = argv[i];
-		size_t pos = arg.find(".yaml");
-		if (pos != string::npos) yaml_files.push_back(arg);
-		pos = arg.find(".yml");
-		if (pos != string::npos) yaml_files.push_back(arg);
+		// Match a trailing .yaml/.yml extension, not any substring, so option values such as
+		// -prefix=run.yaml.bak are not mistaken for input files (and silently dropped).
+		if (ends_with(arg, ".yaml") || ends_with(arg, ".yml")) yaml_files.push_back(arg);
 	}
 	return yaml_files;
 }
@@ -310,6 +313,11 @@ void GOptions::setOptionsValuesFromYamlFile(const std::string& yaml) {
 	try {
 		config = YAML::LoadFile(yaml);
 	}
+	catch (YAML::BadFile& e) {
+		cerr << FATALERRORL << "Cannot open yaml file " << YELLOWHHL << yaml << RSTHHR
+			<< ". Check the path and spelling." << endl;
+		exit(EC__YAML_PARSING_ERROR);
+	}
 	catch (YAML::ParserException& e) {
 		cerr << FATALERRORL << "Error parsing " << YELLOWHHL << yaml << RSTHHR
 			<< " yaml file." << endl;