PostgreSQL compatibility: `round(float8)` should match PostgreSQL half-tie behavior

[Dandandan]

Split out from #22247.

Mismatch

round(float8) should match PostgreSQL half-tie behavior (banker's rounding / round-half-to-even).

SELECT round(2.5::float8);

PostgreSQL:

2

DataFusion:

3.0

Expected behavior

For PostgreSQL-compatible SQL semantics, DataFusion should either match PostgreSQL's result or raise the same class of domain/semantic error.