Change the fragmentId to be a required property in WidgetStateManager

[LukasMasuch]

Describe your changes

The fragmentId is currently an optional property for all the set state methods in the WidgetStateManager. The reason here is that it sometimes has a value and sometimes is just undefined. But this makes it a bit too easy to forget to add it -> which can lead to features that are not compatible with fragments.

This PR changes the property to required by changing from fragmentId?: string to fragmentId: string | undefined. This also uncovered a potentially existing issue with form submission & fragments, which will be fixed via this PR, e.g.:

https://github.com/streamlit/streamlit/pull/8533/files#diff-b6b2b8d7411dc40d04d4dc0442572c4835e84a79844cd2e299a469b7b2ec1a1cR309

Testing Plan

• Update tests

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[vdonato]

Hm, so I think it'd probably be a good idea to convert this PR into a draft for now and pick up the work again when removing the experimental_ prefix from the fragment decorator. We'll need to do some coordination work with other teams to ensure that this doesn't end up resulting in any backwards compatibility issues, and it'll be easy to focus on that when working on other fragment-relate tasks.

Happy to take this PR over once I start working on fragment de-experimentalization in about a month or so.