Fix missing RowDescription when Describe is skipped

In the extended query protocol, Describe is optional. Some clients
(like Fivetran) skip Describe and go directly from Bind to Execute.
The server was not sending RowDescription in this case, causing
"Received resultset tuples, but no field structure for them" errors.

Now Execute checks if Describe was called (via the portal.described
flag) and sends RowDescription if needed before sending data rows.

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

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

Author: EDsCODE

server/conn.go

@@ -1176,7 +1176,14 @@ func (c *clientConn) handleExecute(body []byte) {
 		typeOIDs[i] = getTypeInfo(ct).OID
 	}
 
-	// Don't send RowDescription here - it should come from Describe
+	// Send RowDescription if Describe wasn't called before Execute.
+	// Some clients skip Describe and go straight to Execute, but still
+	// need the column metadata before receiving data rows.
+	if !p.described {
+		if err := c.sendRowDescription(cols, colTypes); err != nil {
+			return
+		}
+	}
 
 	// Send rows with the format codes from Bind
 	rowCount := 0