Block Variation: Block-scoped variations are searchable with the slash inserter

[t-hamano]

#73823 introduced the following new logic to make heading-level variations searchable only with the slash inserter:

It also modifies the behavior of "block" scope in variations so that they are searchable via slash commands.

This means variations with only 'block' scope (not 'inserter') are marked as isSearchOnly and filtered from the inserter browse view, but remain searchable in inserter and slash commands.

This is causing unintended variations to be exposed. For example, all variations of the Columns block can be inserted via the quick inserter:

We need to explore a different approach.