sql/importer: add hint to vector index IMPORT INTO error

Previously, the error message for attempting IMPORT INTO on tables
with vector indexes only stated that the operation is not supported.
This change adds a helpful hint suggesting users drop the vector index
before importing and recreate it afterwards.

The hint provides a clear workaround for users encountering this
limitation, improving the user experience when working with vector
indexes and bulk imports.

Release note: none

Informs: #145227

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mw5h

pkg/sql/importer/import_planning.go

@@ -650,8 +650,9 @@ func importPlanHook(
 		// Check if the table has any vector indexes
 		for _, idx := range found.NonDropIndexes() {
 			if idx.GetType() == idxtype.VECTOR {
-				return unimplemented.NewWithIssueDetail(145227, "import.vector-index",
-					"IMPORT INTO is not supported for tables with vector indexes")
+				return errors.WithHint(unimplemented.NewWithIssueDetail(145227, "import.vector-index",
+					"IMPORT INTO is not supported for tables with vector indexes"),
+					"Consider dropping the vector index before importing, then recreating it afterwards.")
 			}
 		}