fix: MultipleChoiceField use ordered sort

[fbozhang]

This modification has the following advantages:

1. Maintain the input order.
2. Prevent the output from being a set, which would cause issues with JSON serialization (e.g., when passing parameters to certain third-party SDKs that may internally use json.dumps, and I cannot modify the third party's encoder).

[fbozhang]

please also add unit tests to verify this and make the CI green

I’ve added the tests and the CI is green now.

[fbozhang]

Seems sensible to me 👍🏻

However, it will have to wait for 3.17 as I see this as potentially disruptive for some downstream users. Left a small suggestion

Good point — waiting for 3.17 sounds like the right call. Appreciate the suggestion!

[stale[bot]]

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