fix: tactic completion in empty by blocks

[mhuisi]

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.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 96d502bd11cc96bd92dcb6b73e1c4002526647ca --onto 4f6bcc5adac8d6234a9e3def3675402cb89823c6. You can force Mathlib CI using the force-mathlib-ci label. (2026-04-09 16:30:36)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 96d502bd11cc96bd92dcb6b73e1c4002526647ca --onto 861bc19e0c1c45b5766cc5c9ef0488568368b236. You can force reference manual CI using the force-manual-ci label. (2026-04-09 16:30:37)