Fix described flag set before RowDescription is actually sent

The described flag was being set unconditionally at the start of
Describe portal handling, but then NoData might be sent instead of
RowDescription (for non-SELECT, failed describe, or empty columns).

This caused Execute to skip sending RowDescription (thinking Describe
already sent it), resulting in DataRows without field structure error:
"Received resultset tuples, but no field structure for them"

Now described=true is only set when we actually send RowDescription.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: EDsCODE

server/conn.go

@@ -1068,7 +1068,6 @@ func (c *clientConn) handleDescribe(body []byte) {
 			c.sendError("ERROR", "34000", fmt.Sprintf("portal %q does not exist", name))
 			return
 		}
-		p.described = true
 
 		// For non-SELECT, send NoData
 		upperQuery := strings.ToUpper(strings.TrimSpace(p.stmt.query))
@@ -1105,6 +1104,9 @@ func (c *clientConn) handleDescribe(body []byte) {
 			return
 		}
 
+		// Only mark as described when we actually send RowDescription.
+		// If we sent NoData above, Execute should still send RowDescription.
+		p.described = true
 		c.sendRowDescription(cols, colTypes)
 
 	default: