server: handle Describe Portal for declared cursors (psycopg3 compat)

psycopg3's ServerCursor sends Describe Portal with the cursor name
after DECLARE CURSOR. In PostgreSQL, declared cursors are accessible
as portals, but DuckGres stores them separately in c.cursors.

When Describe Portal can't find the name in c.portals, fall back to
checking c.cursors and return the cursor's schema. This is needed for
psycopg3's ServerCursor.execute() flow which describes the cursor
portal to get column metadata before fetching rows.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Sprite

server/conn.go

@@ -3580,6 +3580,19 @@ func (c *clientConn) handleDescribe(body []byte) {
 		// Describe portal
 		p, ok := c.portals[name]
 		if !ok {
+			// In PostgreSQL, DECLARE CURSOR creates a named cursor that is also
+			// accessible as a portal. psycopg3's ServerCursor sends Describe Portal
+			// with the cursor name after DECLARE. Check c.cursors as fallback.
+			if _, cursorOk := c.cursors[name]; cursorOk {
+				cols, colTypes, err := c.getCursorSchema(name)
+				if err != nil {
+					slog.Debug("Describe cursor-as-portal failed to open.", "user", c.username, "cursor", name, "error", err)
+					_ = writeNoData(c.writer)
+					return
+				}
+				_ = c.sendRowDescription(cols, colTypes)
+				return
+			}
 			c.sendError("ERROR", "34000", fmt.Sprintf("portal %q does not exist", name))
 			return
 		}