feat: keep input prompt when switching task types

[edward-ly]

Resolves#217.

Continuing discussion from #227:

The only thing that is not working is the preservation of the "input" input because the default values are applied in 2 places:

• AssistantTextProcessingForm::onTaskTypeUserChange (where the input is preserved)

• AssistantFormInputs::watch::selectedTaskType

and if the second one happens after the first, the input field is cleared.

I'm not sure if AssistantFormInputs::watch::selectedTaskType has any purpose anymore, now that I think about it, especially now with the addition of the TranslateForm. Removing it fixed the issue for me.

[janepie]

As this watch sets the defaults, we still need it (or an alternative), for example for image generation

[janepie]

On further investigation, this should be done in AssistantTextProcessingForm::onTaskTypeUserChange as well, but for some reason the old task type is used there, not the recently chosen one

[janepie]

Looks good to me so far. It doesn't work with the document generation task types as their input is labeled text, but maybe that should be changed in the task types, wdyt @julien-nc ?

[edward-ly]

It doesn't work with the document generation task types as their input is labeled text, but maybe that should be changed in the task types, wdyt @julien-nc ?

Similarly, maybe source_input in context write could be renamed to just input as well? I can add a case for the text parameter in the meantime just like with source_input.

[julien-nc]

I think it's fine like it is now. We can't change input shape keys of task types that were already in some releases, it would break loading the existing tasks that were created with the old keys.