perf: revert rootContext to val for performance optimization

Reverted rootContext back from a method to an immutable val in FeelEngine
to avoid repeated context creation on every evaluation invocation.
The mutable failureCollector is initialized on demand when merging the context.

Partly reverts 851a743

Reviewed-by: saig0
(cherry picked from commit 93dab53)

Author: asigner

src/main/scala/org/camunda/feel/FeelEngine.scala

@@ -133,7 +133,7 @@ class FeelEngine(
       s"configuration: $configuration]"
   )
 
-  private def rootContext(): EvalContext = EvalContext.create(
+  private val rootContext: EvalContext = EvalContext.create(
     valueMapper = valueMapper,
     functionProvider = FunctionProvider.CompositeFunctionProvider(
       List(
@@ -330,7 +330,8 @@ class FeelEngine(
   @deprecated def evaluate(expression: ParsedExpression, context: Context): EvaluationResult =
     Try {
       validate(expression) match {
-        case Right(_)      => eval(expression, rootContext().merge(context))
+        case Right(_)      =>
+          eval(expression, EvalContext.empty(valueMapper).merge(rootContext).merge(context))
         case Left(failure) => FailedEvaluationResult(failure = failure)
       }
     }.recover(failure =>