fix: resolve Goroutine leak in database connection close (apache#3491)

When a connection is opened via databaseImpl, the cache clientCache
initializes
*flightsql.Client objects which require proper cleanup on connection
close to
prevent Goroutine leaks.

Previously, closing a connection released only 3 of the 6 Goroutines
created per
connection, leaving 3 Goroutines associated with the cached client
unmanaged.
This resulted in accumulated Goroutine leaks over time.

The fix ensures all Goroutines are properly cleaned up when connections
are closed.

Author: bruceNu1l

go/adbc/driver/flightsql/flightsql_connection.go

@@ -1131,6 +1131,7 @@ func (c *connectionImpl) Close() error {
 		}
 	}
 
+	c.clientCache.Purge()
 	err = c.cl.Close()
 	c.cl = nil
 	return adbcFromFlightStatus(err, "Close")

go/adbc/driver/flightsql/flightsql_database.go

@@ -494,7 +494,13 @@ func (d *databaseImpl) Open(ctx context.Context) (adbc.Connection, error) {
 			if err != nil {
 				d.Logger.Debug("failed to close client", "error", err.Error())
 			}
-		}).Build()
+		}).PurgeVisitorFunc(func(_ interface{}, client interface{}) {
+		conn := client.(*flightsql.Client)
+		err := conn.Close()
+		if err != nil {
+			d.Logger.Debug("failed to close client", "error", err.Error())
+		}
+	}).Build()
 
 	var cnxnSupport support