feat(index): support dry_run in drop_global_index

[JunRuiLee]

Stacked on #487 — this branch builds on feat/global-index-drop-lumina-vindex. Until #487 merges, the diff below also shows #487's commits; it will narrow to just the dry_run commit once #487 is merged. Kept as draft until then.

Purpose

Add dry_run to drop_global_index, matching Java's #8309. A dry run reports how many global index files a drop would remove, committing nothing — a safety preview before the irreversible metadata commit. Because it lives in the shared GlobalIndexDropBuilder, it covers all supported types (btree/bitmap/lumina/vindex) in one change.

Changes

• GlobalIndexDropBuilder: new with_dry_run(bool); execute() still returns the matched count (usize), but on dry-run short-circuits before the commit, so nothing is written. The non-dry-run path is unchanged.
• sys.drop_global_index: parses the dry_run arg (true/false, case-insensitive; invalid → error), and on dry-run returns Would drop N global index file(s) instead of committing.

Out of scope

partitions remains NotImplemented (separate follow-up).

Tests

• Builder: dry-run returns the matched count and leaves the index entry in place (no commit), then a real drop removes it; dry-run with no match returns 0.
• Procedure: drop_global_index(... dry_run => true) leaves $table_indexes unchanged, then a real drop empties it; invalid dry_run value is rejected.