fix: allow query().current and other data properties to work in non-reactive contexts

[ottomated]

As of #15533, query().current no longer works in non-reactive contexts. I believe this is unwanted behavior -- take the following scenario:

<script>
  import { list_todos } from '$lib/todos.remote';
  $effect(() => {
    const events = subscribe_to_something_external();
    events.on('new-todo', (item) => {
      // I don't want to call `.run()` here, because if `list_todos`
      // is not currently used, I don't care about the result
      // (it would just waste a network request)
      const current = list_todos().current;
      if (!current) return;
      list_todos().set([data.item, ...current]);
    });

  });
</script>

This PR fixes it by simply restoring the old behavior of .current returning undefined if the query has not been called yet.

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Preview: https://svelte-dev-git-preview-kit-15699-svelte.vercel.app/

[elliott-with-the-longest-name-on-github]

This is intentional. There's a pretty good explanation here: https://svelte.dev/docs/kit/remote-functions#query-Deduplication

Basically, because .current (and the other data properties) interact with data that's in the remote functions cache, they need an "anchor" in the reactive context (otherwise we have no way of managing the cached data). I think the correct solution here might just be to give .set a callback overload so that you can access the current value (if any), but only within the context of setting it:

query.set((current) => [new, ...current]);

Since set is already a noop if there's nothing listening for it, it'd achieve what you want. (It's also weird that, as written in this PR, you can access .current but none of the other data-related props.)

[ottomated]

@elliott-with-the-longest-name-on-github I'm not sure I understand fully – is it not possible to just get the cached value if it exists and return undefined if it doesn't? This patch is working as expected for me, so is there a hidden bug I'm not seeing?

A callback form of .set would definitely also work, yes.

[elliott-with-the-longest-name-on-github]

Ok we've discussed this at length and I think this is the right way to go, but if we're doing it with current we need to do it with error/loading/ready as well. Mind making that update? loading and ready should both fall back to false, for obvious reasons

[ottomated]

I've added the other properties. I'll make a new PR for the callback form of set tomorrow.

[elliott-with-the-longest-name-on-github]

@ottomated we can leave the new .set API off for now -- I think accessing the properties on uncached instances provides all of the benefits with additional flexibility. We can always add the .set API later if there's demand for it.