fix(model-picker): resolve TimeoutError when selecting current model

[cdakotabrown]

• Convert interactive_model_picker from async to sync using arrow_select()
• Remove ThreadPoolExecutor+asyncio.run pattern that caused TimeoutError
• Extract build_model_choices() and parse_model_choice() helpers
• Move formatting constants to model_picker_constants.py
• DRY up update_model_in_input() with _process_model_command() helper
• Reduce spinner pause from 300ms to 50ms (matches other menus)
• Add 30 unit tests for model picker functions

The bug occurred because asyncio.run() in ThreadPoolExecutor created
event loop conflicts. The sync arrow_select() works cleanly without
these complications.

[coderabbitai]

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[cdakotabrown]

Need to test this on my home machine before this is ready for review