Refactor ask_user interactions

[F16shen]

Summary

• migrate ask_user onto a structured interaction request/response model
• support single_select, text_input, and choice_or_text through the same interaction pipeline
• remove the obsolete inline ask_user path and update docs/tests for the new schema

Testing

• pytest tests/test_shell.py tests/test_shell_prompt_io.py tests/test_interaction_ask_user.py tests/test_ask_user_tool.py -q

Follow-up

• text_input default handling should be fixed in a follow-up PR
• InteractionService TTY gating should be revisited to avoid short-circuiting shell fallback behavior

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