fix(compiler): preserve nullability when casting JSON arrow expressions (#3792)

[luongs3]

Fixes #3792.

Problem

PostgreSQL's JSON accessor operators (->, ->>, #>, #>>) return SQL NULL when the requested key or path is missing from the value. Wrapping such an expression in a type cast does not make the result non-nullable.

Today sqlc emits a non-nullable Go string for the issue's exact query:

-- name: ListJobs :many
SELECT id,
       (data ->> 'PhoneNumber')::text AS phone_number,
       (data ->> 'ContactName')::text AS contact_name,
       (data ->> 'State')::text       AS state
FROM jobs;

Generated row before this fix:

type ListJobsRow struct {
    ID          int64
    PhoneNumber string
    ContactName string
    State       string
}

Generated row after:

type ListJobsRow struct {
    ID          int64
    PhoneNumber pgtype.Text
    ContactName pgtype.Text
    State       pgtype.Text
}

Any consumer scanning into pgtype.Text / *string / sql.NullString previously crashed on rows where the JSON key happened to be absent. Real Postgres returns NULL in that case; the generated row now matches.

Fix

Two small changes:

• internal/sql/lang/operator.go — new IsJSONNullableOperator(s string) bool classifier covering ->, ->>, #>, #>>.
• internal/compiler/output_columns.go — in the *ast.TypeCast case, when the wrapped argument is an *ast.A_Expr whose operator is JSON-nullable, set col.NotNull = false. Tight scope: only the immediate cast argument. All other cast paths (literal cast, column-from-NOT-NULL cast, etc.) are unchanged.

Broader nullability inference for casts is intentionally out of scope (the existing // TODO Add correct, real type inference comment in the cast handler stays — this PR addresses the specific JSON-arrow case from #3792 without expanding into general expression-tree analysis).

Tests

• internal/sql/lang/operator_test.go — TestIsJSONNullableOperator table-driven test, positive (->, ->>, #>, #>>) and negative (+, -, =, @>, <@, ->>> lookalike, empty).
• internal/endtoend/testdata/json_arrow_cast_3792/postgresql/pgx/v5/ — end-to-end fixture reproducing the issue's exact query under pgx/v5; golden files show pgtype.Text for the three ->>-derived columns.

Verified locally

• Issue repro generates pgtype.Text instead of string (the bug).
• ('foo')::text and (name)::text where name is NOT NULL still emit string — no regression on the common cast cases.
• (NULL)::text still emits pgtype.Text (preserves the existing NULL-literal-cast carve-out).
• (data -> 'k')::jsonb correctly flows through the same path.
• go test ./internal/sql/... ./internal/compiler/... ./internal/engine/... passes; gofmt -l and go vet clean on touched files.