fix: cast outer projection expr to declared field type in SingleDistinctToGroupBy

[jdymitarai]

Closes #26193.

When SingleDistinctToGroupBy rewrites an aggregate containing both DISTINCT and non-distinct expressions such as sum over Decimal128, the non-distinct expression is split into inner and outer phases. The outer phase evaluates sum over the already-widened Decimal output of the inner aggregation, which widens precision a second time and causes the resulting schema to diverge from the original aggregate schema.

To maintain schema consistency, the outer projection now checks if the data type produced by outer_aggr differs from the field type declared in the original schema, wrapping the projected expression with a cast back to the declared field type when a mismatch occurs.

All optimizer unit tests including a dedicated regression test reproducing the two-phase decimal precision expansion pass cleanly.