This PR fixes a bug where tactic auto-completion would produce tactic
completion items in the entire trailing whitespace of an empty tactic
block. Since #13229 further restricted top-level `by` blocks to be
indentation- sensitive, this PR adjusts the logic to only display
completion items at a "proper" indentation level.
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
