isthmus: CEIL, FLOOR and ROUND over a decimal emit an output type their declaration does not derive #1349

SqlToSubstrait maps Calcite's CEIL, FLOOR and ROUND over a decimal to ceil:dec, floor:dec and round:dec_i32 in functions_rounding_decimal.yaml, but writes the output type Calcite infers, not the one the declaration derives. For ROUND the emitted type is narrower than declared, so a value that rounds up to the next magnitude does not fit it: ROUND(99999.99, 0) is 100000.00, which needs decimal(8,2), but the plan says decimal(7,2).

CREATE TABLE t (d DECIMAL(7, 2) NOT NULL, z DECIMAL(7, 0) NOT NULL);
SELECT CEIL(d), FLOOR(z), ROUND(d, 1) FROM t;

"Declared" is resolveType over the arguments the plan actually emits. CEIL/FLOOR declare decimal<min(P - S + 1, 38), 0>, and round declares decimal?<min(P + 1, 38), S> with DECLARED_OUTPUT, so its result is nullable even for a required argument.

Measured on main at bac6b43 (substrait-packaging 0.102.0, Calcite 1.42.0).

Unlike the decimal arithmetic in #1347, SubstraitTypeSystem cannot fix this. Calcite fixes these return types on the operators themselves (SqlFloorFunction uses ReturnTypes.ARG0_OR_EXACT_NO_SCALE, SqlStdOperatorTable.ROUND uses ReturnTypes.ARG0_NULLABLE), and no RelDataTypeSystem hook is consulted. Fixing it means isthmus-local CEIL/FLOOR/ROUND operators that derive the declared types. round:dec_i32 already shows up in the TPC-H/TPC-DS sweep in #1117 (as "mostly nullability"); ceil/floor do not, because no TPC query uses them over a decimal.