fix: replace string matching with getMessageCode in SpEL exception handling for improved clarity

- Used `METHOD_NOT_FOUND` constant instead of string matching for detecting method-not-found errors in SpEL expressions.

Author: predic8

core/src/main/java/com/predic8/membrane/core/lang/spel/SpELExchangeExpression.java

@@ -30,6 +30,7 @@
 
 import static java.lang.Boolean.*;
 import static org.springframework.expression.spel.SpelCompilerMode.*;
+import static org.springframework.expression.spel.SpelMessage.METHOD_NOT_FOUND;
 
 public class SpELExchangeExpression extends AbstractExchangeExpression {
 
@@ -84,7 +85,7 @@ public <T> T evaluate(Exchange exchange, Flow flow, Class<T> type) {
  } catch (SpelEvaluationException see) {
  ExchangeExpressionException eee = new ExchangeExpressionException(expression, see);
  String msg ;
- if (see.getMessage().contains("EL1004E")) {
+ if (see.getMessageCode() == METHOD_NOT_FOUND) {
  msg = "Method not found in expression '%s' use a SpEL function or one of Membrane's: %s".formatted(expression, SpELBuiltInFunctions.getBuiltInFunctionNames());
  } else if (see.getCause() instanceof ConverterNotFoundException cnfe) {
  msg = "Type converter not found for expression '%s' from '%s' to '%s'.".formatted(expression, cnfe.getSourceType(), cnfe.getTargetType());