UPDATE decimal_col = float_col rounds last digit up by 1 ULP (regression test working_dml/misc/bug3350.sql fails)

Summary

UPDATE <decimal_col> = <float_col> rounds the last decimal place up by 1 ULP. Regression — does not happen before MCOL-4628 (d101692e7, 2026-04-29).

Repro

CREATE TABLE t (h DECIMAL(10,4), f FLOAT) ENGINE=ColumnStore;

INSERT INTO t VALUES (1234.56, 1234.56);

UPDATE t SET h=f;

SELECT h, f FROM t;

-- expected: h=1234.5600  f=1234.56

-- actual:   h=1234.5601  f=1234.56

Verified on mariadb-columnstore 25.10.4 (already includes the MCOL-4628 backport).

Affected regression test

Nightly suite test005.sh → working_dml/misc/bug3350.sql — fails on all 18 stages of build #3564 (every distro × every server version × ASan/UBSan) with the same one-row diff in column h DECIMAL(10,4) after UPDATE bug3350 SET ..., h=f, ....