fix(input_select): Decouple input_select() and input_selectize()

[ErdaradunGaztea]

Fixes #1687

This PR follows the suggestions made in the issue and repurposes current input_select() implementation as _input_select_impl(). The new input_select() keeps the same signature but raises deprecation warnings upon using any of the three Selectize-related parameters.

Additionally, passing anything that's not a string (i.e. a HTML tag or None) as an option label raises another deprecation warning. The final goal would be to replace _Choices = Mapping[str, TagChild] with _Choices = Mapping[str, str], but I guess that comes after a period of deprecation.

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It's my first PR to this project, so if I missed some important part of the PR process, please let me know.

[schloerke]

It's my first PR to this project

Thank you for your PRs!

Please be sure to add an entry in the CHANGELOG.md file linking to your PR. (Related example)

[schloerke]

The app test failing in https://github.com/posit-dev/py-shiny/actions/runs/14320952734/job/40142214665?pr=1947#step:5:1519 is due to a change in this PR.

pytest ./tests/playwright/ai_generated_apps/bookmark/input_select

Please update the test app or fix the helper method. Thank you!

[ErdaradunGaztea]

The app test failing in https://github.com/posit-dev/py-shiny/actions/runs/14320952734/job/40142214665?pr=1947#step:5:1519 is due to a change in this PR.

pytest ./tests/playwright/ai_generated_apps/bookmark/input_select

Please update the test app or fix the helper method. Thank you!

I made a mistake in the code, that's why it wasn't passing; fixed that. I've addressed all your other points, too. Please tell me if the updated approach with the DEPRECATED sentinel is how you do it, because there aren't any examples in the code to compare to (navbar_options_resolve_deprecated() is the only one).