Apply code simplification improvements

- Consolidated two contains() methods into single template function
- Added const correctness to helper methods in argument_parser.h
- Improved string handling (replaced substr with compare, return "" instead of std::string())
- Fixed indentation in nlsat_solver.cpp nested loop
- Modernized empty container return syntax in lp.cpp

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

src/test/lp/lp.cpp

@@ -732,7 +732,7 @@ vector<std::string> get_file_names_from_file_list(const std::string& filelist) {
     std::ifstream file(filelist);
     if (!file.is_open()) {
         std::cout << "cannot open " << filelist << std::endl;
-        return vector<std::string>();
+        return {};
     }
     vector<std::string> ret;
     bool end;