Fixes #15707 :

<form {...test_form}>
	// not displaying 'default text'
	<p>{test_form.fields.name.value()}</p>

	// displays 'default text'
	<input {...test_form.fields.name.as('text', 'default text')} />
</form>

When calling fields.name.as('text', 'default'), the input_value was used as a display fallback in the returned props but was never accessible to fields.name.value(), which returned undefined until the user interacted with the input.

When .as() is called with an input_value, it now writes it to the cache via set_input, so get_value() can immediately return it.

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