fix(databricks): emit only changed column-tag keys in SET TAGS diffs

[sd-db]

Summary

• Port dbt-databricks#1668: for a column whose tags changed, ALTER COLUMN … SET TAGS now includes only new or updated keys, not that column's full desired map.
• Unchanged columns are still omitted. Newly tagged columns still receive the full desired map.
• Set-only semantics are unchanged: remote-only keys are never unset.
• Follow-up to #16247, which skips unchanged columns but still sends the full map for a changed column.

Fixes #16272

Test plan

• cargo test -p dbt-adapter test_get_diff_ (including test_get_diff_omits_unchanged_keys_within_column)
• CI on this PR