[C++] Disallow trailing commas in function call argument lists in Pratt parser

According to the CEL grammar specification, trailing commas are only permitted in collection literals (lists, maps, structs), not in function call argument lists (`exprList`).

Disallows trailing commas in `PrattParserWorker::ParseArguments` and adds test cases to verify syntax error reporting.

PiperOrigin-RevId: 972728668

Author: dmitriplotnikov

parser/internal/pratt_parser_test.cc

@@ -1283,6 +1283,12 @@ std::vector<ErrorTestCase> GetErrorTestCases() {
               " | f(1, 2\n"
               " | ......^",
       },
+      ErrorTestCase{
+          .source = "foo(a,b,)",
+          .expected_error = "ERROR: <input>:1:9: unexpected token\n"
+                            " | foo(a,b,)\n"
+                            " | ........^",
+      },
       ErrorTestCase{
           .source = "999999999999999999999999999999999999999",
           .expected_error = "ERROR: <input>:1:1: invalid int literal\n"

parser/internal/pratt_parser_worker.h

@@ -852,6 +852,7 @@ std::vector<ExprNode> PrattParserWorker<ExprNode>::ParseArguments(
       if (peek_token_.type == TokenType::kComma) {
         NextToken();
         if (peek_token_.type == close_token) {
+          ReportError(peek_token_, "unexpected token");
           break;
         }
         continue;

parser/parser_test.cc

@@ -501,6 +501,17 @@ std::vector<TestInfo> test_cases = {
      "ERROR: <input>:1:3: expected struct field name\n"
      " | t{>C}\n"
      " | ..^"},
+    {"foo(a,b,)", "",
+     "ERROR: <input>:1:9: Syntax error: mismatched input ')' expecting "
+     "{'[', '{', '(', '.', '-', '!', 'true', 'false', 'null', NUM_FLOAT, "
+     "NUM_INT, NUM_UINT, STRING, BYTES, IDENTIFIER}\n"
+     " | foo(a,b,)\n"
+     " | ........^",
+     "", "", "", "",
+     // PRATT PARSER ERROR MESSAGE
+     "ERROR: <input>:1:9: unexpected token\n"
+     " | foo(a,b,)\n"
+     " | ........^"},
 
     // Macro tests
     {"has(m.f)", "m^#2:Expr.Ident#.f~test-only~^#4:Expr.Select#", "",