Complete Flows can be created that have no streaming result

[loukratz-bv]

Example:

  - type: Flow
    id: tool_invocation_example
    interface:
      type: Complete
    variables:
      - id: input_msg
        type: text
      - id: tool_result
        type: text
    inputs:
      - input_msg
    outputs:
      - tool_result
    steps:
      - id: tool_step
        type: InvokeTool
        tool: q_example_qtype.tools.my_function_1
        inputs:
          - input_msg
        outputs:
          - tool_result
        input_bindings:
          msg: input_msg
        output_bindings:
          result: tool_result

This is a bad user experience -- the validation should catch this or something