Support SpEL compilation of #root or #this with non-public type

Prior to this commit, if a Spring Expression Language (SpEL) expression
referenced the root context object via the #root or #this variable, we
inserted a checkcast in the generated byte code that cast the object to
its concrete type. However if the root context object's type was
non-public, that resulted in an IllegalAccessError when the compiled
byte code was executed.

VariableReference.getValueInternal() already contains a solution for
global variables which inserts a checkcast to Object in the generated
byte code instead of to the object's concrete non-public type.

This commit therefore applies the same logic to #root (or #this when
used to reference the root context object) that is already applied to
global variables.

Closes gh-32356

framework-docs/modules/ROOT/pages/core/expressions/language-ref/variables.adoc

@@ -20,6 +20,15 @@ characters.
 * dollar sign: `$`
 ====
 
+[TIP]
+====
+When setting a variable or root context object in the `EvaluationContext`, it is advised
+that the type of the variable or root context object be `public`.
+
+Otherwise, certain types of SpEL expressions involving a variable or root context object
+with a non-public type may fail to evaluate or compile.
+====
+
 [WARNING]
 ====
 Since variables share a common namespace with