Initialize ScanningRecipe accumulator key eagerly for thread safety

[pstreef]

Problem

ScanningRecipe.recipeAccMessage uses lazy initialization that is not thread-safe:

@Nullable
private String recipeAccMessage;

private String getRecipeAccMessage() {
    if (recipeAccMessage == null) {
        recipeAccMessage = "org.openrewrite.recipe.acc." + UUID.randomUUID();
    }
    return recipeAccMessage;
}

When recipe instances are shared across concurrent runs, two threads can see null simultaneously, generate different UUIDs, and one overwrites the other. The losing thread's accumulator data — stored under the now-orphaned UUID — becomes unreachable.

Solution

Change to eager initialization at field declaration. The UUID generation cost is negligible and the field is always used, so lazy init provided no benefit. The clone() method continues to assign a new UUID as before.