Fix #1029: correct left-associativity for arithmetic operators in script parser

The scripting engine evaluated "A - B + C" as "A - (B + C)" instead of
"(A - B) + C". The root cause was that string_concat's operand chain
referenced math_sum, causing it to appear twice in lexy's operation list
with conflicting binding power levels. Changed string_concat::operand
from math_sum to math_prefix to eliminate the duplicate entry.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: facontidavide

include/behaviortree_cpp/scripting/operators.hpp

@@ -747,7 +747,10 @@ struct Expression : lexy::expression_production
       return dsl::op<Ast::ExprBinaryArithmetic::concat>(LEXY_LIT(".."));
     }();
 
-    using operand = math_sum;
+    // Use math_prefix (not math_sum) to avoid duplicating math_sum/math_product
+    // in the operation list, which would break left-associativity of +/-/*//
+    // due to conflicting binding power levels in lexy's _operation_list_of.
+    using operand = math_prefix;
   };
 
   // ~x

tests/script_parser_test.cpp

@@ -424,6 +424,42 @@ TEST(ParserTest, Issue595)
   ASSERT_EQ(0, counters[0]);
 }
 
+// https://github.com/BehaviorTree/BehaviorTree.CPP/issues/1029
+TEST(ParserTest, OperatorAssociativity_Issue1029)
+{
+  BT::Ast::Environment environment = { BT::Blackboard::create(), {} };
+
+  auto GetResult = [&environment](const char* text) -> BT::Any {
+    return GetScriptResult(environment, text);
+  };
+
+  // Addition and subtraction are left-associative:
+  // "5 - 2 + 1" should be (5-2)+1 = 4, NOT 5-(2+1) = 2
+  EXPECT_EQ(GetResult("5 - 2 + 1").cast<double>(), 4.0);
+
+  // "10 - 3 - 2" should be (10-3)-2 = 5, NOT 10-(3-2) = 9
+  EXPECT_EQ(GetResult("10 - 3 - 2").cast<double>(), 5.0);
+
+  // "2 + 3 - 1" should be (2+3)-1 = 4
+  EXPECT_EQ(GetResult("2 + 3 - 1").cast<double>(), 4.0);
+
+  // Multiplication and division are also left-associative:
+  // "12 / 3 / 2" should be (12/3)/2 = 2, NOT 12/(3/2) = 8
+  EXPECT_EQ(GetResult("12 / 3 / 2").cast<double>(), 2.0);
+
+  // "12 / 3 * 2" should be (12/3)*2 = 8, NOT 12/(3*2) = 2
+  EXPECT_EQ(GetResult("12 / 3 * 2").cast<double>(), 8.0);
+
+  // Mixed precedence: "2 + 3 * 4 - 1" should be 2+(3*4)-1 = 13
+  EXPECT_EQ(GetResult("2 + 3 * 4 - 1").cast<double>(), 13.0);
+
+  // Verify string concatenation operator (..) still works after operand fix
+  EXPECT_EQ(GetResult("A:='hello'; B:=' world'; A .. B").cast<std::string>(), "hello "
+                                                                              "world");
+  // Chained concatenation (left-associative)
+  EXPECT_EQ(GetResult("A .. ' ' .. B").cast<std::string>(), "hello  world");
+}
+
 TEST(ParserTest, NewLine)
 {
   BT::BehaviorTreeFactory factory;