Fix SET statement behavior differences between server and memory engines

The issue was two-fold:

1. SET statements used an empty schema instead of OkResultSchema, causing
   the server handler to process them through the wrong code path.

2. In server engine tests, SET statements were routed to the query() path
   instead of the exec() path, causing them to return empty result sets
   instead of OkResult.

Changes:
- sql/plan/set.go: Use types.OkResultSchema instead of empty schema
- enginetest/server_engine.go: Remove SET from shouldQuery list so it
  goes through exec() path which properly handles OkResult

This ensures SET statements return consistent OkResult behavior in both
memory engine and server engine modes, fixing CI test failures.

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

Co-Authored-By: Claude <[email protected]>

Author: macneale4

enginetest/server_engine.go

@@ -250,7 +250,7 @@ func (s *ServerQueryEngine) queryOrExec(ctx *sql.Context, stmt *gosql.Stmt, pars
 			shouldQuery = true
 		}
 	case *sqlparser.Select, *sqlparser.SetOp, *sqlparser.Show,
-		*sqlparser.Set, *sqlparser.Call, *sqlparser.Begin,
+		*sqlparser.Call, *sqlparser.Begin,
 		*sqlparser.Use, *sqlparser.Load, *sqlparser.Execute,
 		*sqlparser.Analyze, *sqlparser.Flush, *sqlparser.Explain:
 		shouldQuery = true

sql/plan/set.go

@@ -19,6 +19,7 @@ import (
 	"strings"
 
 	"github.com/dolthub/go-mysql-server/sql"
+	"github.com/dolthub/go-mysql-server/sql/types"
 )
 
 // Set represents a set statement. This can be variables, but in some instances can also refer to row values.
@@ -79,7 +80,7 @@ func (s *Set) Expressions() []sql.Expression {
 
 // setSch is used to differentiate from the nil schema,
 // because Set does return rows
-var setSch = make(sql.Schema, 0)
+var setSch = types.OkResultSchema
 
 // Schema implements the sql.Node interface.
 func (s *Set) Schema() sql.Schema {