Fix stale content cache when slots are accessed before render_in

[armstrjare]

Problem

content caches its result in @__vc_content after first evaluation. render_in resets @__vc_content_evaluated to false, but doesn't clear the cached @__vc_content value. This means if content was evaluated before render_in, the stale cached result is returned on every subsequent call — the render block passed when rendering the component is never evaluated.

This is easy to trigger in practice because __vc_get_slot eagerly calls content to ensure content-defined slots are loaded:

# slotable.rb
def __vc_get_slot(slot_name)
  content unless defined?(@__vc_content_evaluated) && @__vc_content_evaluated
  ...
end

So any slot predicate (e.g. column?, header?) called before render_in poisons the cache. This comes up when building a component's structure first (configuring slots in a helper or setup block) and rendering it later with a content block:

# helper.rb
def my_table
  Ui::Table.new.tap do |table|
    table.with_column "Name" %>
    table.with_column :options unless table.column?(:options) # triggers content evaluation
  end
end

<%# template.html.erb %>
<%= render my_table do %>
  <%= render partial: "row" %>  <%# silently ignored %>
<% end %>

Fix

Clear @__vc_content in render_in alongside the existing @__vc_content_evaluated reset, so the cache is fully invalidated on each render.

Adds a regression test that accesses a slot predicate before render_in and asserts the content block is rendered.