sql: discard final rows from SETOF VOID functions

[Alignyx]

Selecting from a SQL UDF declared RETURNS SETOF VOID currently exposes a NULL row from its final SELECT NULL. This change makes that function produce no result rows, while still executing its body.

The existing DiscardLastStmtResult path already runs the final statement without adding its output to the routine's result buffer. buildUDF now selects that path for a set-returning routine whose result family is VOID, as well as for routines that use an explicit result buffer. Checking both properties keeps scalar VOID functions and non-VOID set-returning functions on their existing paths.

The issue explicitly leaves open whether CockroachDB should adopt PostgreSQL's behavior here. This PR implements that compatibility choice; it does not assume that the API decision has already been made.

CREATE FUNCTION f() RETURNS SETOF VOID LANGUAGE SQL AS 'SELECT NULL';
SELECT f();
-- Before: one NULL row. After: no rows.

The regression exercises calls in both the SELECT list and FROM clause, checks that a volatile function still performs an INSERT, and checks scalar VOID and SETOF INT results.

Validation

• bazel test //pkg/sql:sql_test --test_arg=-test.run=^TestSetReturningVoidCardinality$ passed.
• The same test added to unmodified 8812064a failed because each SETOF VOID call returned [[NULL]] instead of an empty result.
• Candidate and baseline runs used two build jobs, disabled sharding, one Go-test parallel worker, and a fixed seed. CockroachDB's randomized test knobs, test tenants and DRPC were disabled.

The earlier broad //pkg/sql:sql_test run did not complete cleanly on this machine. TestTenantStatementTimeoutAdmissionQueueCancellation also timed out when run alone on the unmodified baseline. Other resource-related failures passed when rerun with one job. I am reporting the focused regression results here, not a clean full SQL-suite pass.

Fixes #103119

Release note (backward-incompatible change): SQL functions declared RETURNS SETOF VOID no longer produce a row from their final body statement. Applications relying on that row should use a scalar VOID function when one return row is required. Function bodies still execute, including their side effects.

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