postgres: infer RETURNING OLD/NEW columns as nullable

[DoTuanAnh2k1]

Why

sqlx::query! infers a non-nullable column for a RETURNING old.<col> clause even when the value is null at runtime, producing an unexpected-null error.

A RETURNING clause can reference the OLD/NEW transition relations (Postgres 18+). Those columns are nullable regardless of the underlying column's NOT NULL constraint:

• OLD is null for a row added by INSERT (including ON CONFLICT DO NOTHING),
• NEW is null for a row removed by DELETE.

The EXPLAIN-based nullability inference only marked outer-join columns as nullable, so RETURNING old.hash on a NOT NULL PRIMARY KEY was reported as non-nullable (see #4332).

What changed

visit_plan now marks any ModifyTable output referencing an old./new. transition relation as nullable, mirroring the existing over-approximation used for outer joins (a false positive here is safe; a false negative causes the runtime error). This required parsing the Node Type field from the EXPLAIN plan.

Testing

Added a unit test built from the exact EXPLAIN (VERBOSE, FORMAT JSON) output in #4332; it fails on the current code ([None]) and passes with the fix ([Some(true)]). cargo test -p sqlx-postgres, cargo fmt --check and cargo clippy -p sqlx-postgres are clean.

Fixes #4332.

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_{Disclosure: prepared with AI assistance.}