Nonmatching column tags in an index can cause index to be removed in merge

If a table is being merged and has different column tags on each branch, an index definition can be silently dropped during the merge if the column tags for the columns covered by the index do not match.

A [skipped test in `dolt_queries_merge.go`](https://github.com/dolthub/dolt/blob/766335e120d74b3e3653381fc133ea1468e7d7c8/go/libraries/doltcore/sqle/enginetest/dolt_queries_merge.go#L2925) repros this issue.

Start with the skipped test in go/libraries/doltcore/sqle/enginetest/dolt_queries_merge.go around line 2925 and run it to reproduce the merge behavior. Trace the merge handling for indexes when column tags differ between branches. Done means the test is enabled and confirms that the index definition is not silently dropped.

Written by the indexing model from the issue text.