feat: show full type in tooltips for hints

[roife]

fix #19615.

[roife]

[Veykril]

This should not be needed here, we should instead record the hover tooltip for the ... parts (to make that work, we probably need to add a function to HirWrite that is called when the truncation string gets emitted).

[roife]

If so, should we concatenate the types at the end? 👀 I tried adding a tooltip only for ..., but it was quite inconvenient. Displaying the tooltip for the entire type directly is a better choice.

[Veykril]

The feature itself shouldn't require changes to our inlay hint structure here (and thus also no changes in to_proto). Associating a tooltip with all parts doesnt make sense when every part can track tooltips themselves.

I would expect that if HirWrite had a function like fn start_truncated(&mut self) (and end) that gets called when the formatting infra emits a ... we could have that implemented as:

    fn start_truncated(&mut self) {
        self.make_new_part();
        // render full type into the new part tooltip here / (lazily)
    }

    fn end_truncated(&mut self) {
        self.make_new_part();
    }

and probably needs InlayHintLabelBuilder to get the full type as an additional field or so