fix: ra inputs outside <Form> need a SourceContext

[adguernier]

Problem

Inputs outside a <Form> now need a SourceContext but sometimes we need to use ra inputs outside a form

Solution

Provide a default SourceContext

[fzaninotto]

As this is the purpose of this PR, could you please add a story and a unit test showing that TextInput works inside a simple <form> tag?

[fzaninotto]

I'd move it to the TextInput story. It doesn't make sense to tests a ra-ui-mui component inside ra-core.

[djhi]

Just use a custom TextInput like in ReferenceInputBase.stories.tsx