Fix nested_call edge case

[kxmbrian]

What

@jrothrock @natematykiewicz - I missed an edge case in #486

The replace action in on_hash should ideally run only for the case where the Ruby 3.1 hash shorthand syntax is used. If the value_node takes in an argument, we can be sure it's not the shorthand syntax case. This PR adds a check for that:

value_node.children.size == 2

Test

Without the change, params["timestamp"] gets dropped:

# Prior to this PR
nested_call: ::ScoutApm::AutoInstrument(\"nested_call: nested_call(params[\\\"timestamp\\\"])\",[\"ROOT/test/unit/auto_instrument/hash_shorthand_controller.rb:15:in `hash'\"]){nested_call}
# Fixed
nested_call: ::ScoutApm::AutoInstrument(\"nested_call(params[\\\"timestamp\\\"])\",[\"ROOT/test/unit/auto_instrument/hash_shorthand_controller.rb:15:in `hash'\"]){nested_call(params[\"timestamp\"])}

leading to errors like wrong number of arguments (given 0, expected 1) (ArgumentError).

[jrothrock]

Nice catch!