Fix duplicate INSERT execution with RETURNING clause

Portal Describe was executing DML queries to get metadata, causing
INSERT/UPDATE/DELETE with RETURNING to run twice. Now uses synthetic
metadata for RETURNING clause and defers RowDescription to Execute
phase for RETURNING * pattern.

Fixes:
- v1.2.19: Prevent Describe Portal from executing DML
- v1.2.20: Handle RETURNING * by sending NoData in Describe
- v1.2.21: Send RowDescription in Execute when Describe sent NoData

Also includes:
- UUID extraction from INSERT for RETURNING emulation (v1.2.16)
- execute_many with RETURNING aggregation (v1.2.17)
- None→NULL parameter binding fix (v1.2.18)

Author: tom-dyar

src/iris_pgwire/__init__.py

@@ -6,7 +6,7 @@
 caretdev/sqlalchemy-iris.
 """
 
-__version__ = "1.2.14"
+__version__ = "1.2.21"
 __author__ = "Thomas Dyar <thomas.dyar@intersystems.com>"
 
 # Don't import server/protocol in __init__ to avoid sys.modules conflicts