Fixes two related input-handling bugs in the onboard wizard:
1. _input_text treated "" as None, preventing users from clearing
optional string fields or entering empty strings intentionally.
2. _input_model_with_autocomplete used `if value else None`, which
discarded falsy values such as empty strings or 0.
To support clearing optional string fields, add _is_str_or_none() and
normalize empty strings to None inside _configure_pydantic_model only
when the field annotation is `str | None`. Required str fields keep
"" as a valid value.
Also included:
- Remember last selected item in provider/channel/model menus for
better UX when configuring multiple items.
- Rename _SIMPLE_TYPES and _MENU_DISPATCH to lowercase to follow
Python naming conventions (they are local variables, not constants).
- Remove unused imports in test file.
Extracted from PR #3358.