[BUG] Multiple swaps allowed in single graph step

[keithmanville]

A task graph with multiple swaps currently passes validation.

For example:

step_name:
  ?math_swap:
    add:
      task: add_numbers
      kwargs:
        num1: $num1
        num2: $num2
    sub:
      task: subtract_numbers
      kwargs:
        num1: $num1
        num2: $num2
  ?string_swap:
    hi:
      task: hello_world
      kwargs:
        greeting: $greeting
        name: $name
    type:
      task: print_type

Update the validation so this error is caught. It should happen in the "light weight" validation so that it runs on explicit validation check and when an entrypoint is saved.