Preserve precision for numeric power functions

[fallintoplace]

Summary

ClickHouse evaluates pow() as Float64, so pushing down PostgreSQL numeric overloads can round values before an exact comparison. Keep float8 pushdown, but evaluate numeric pow() and power() locally in PostgreSQL.

The regression uses a Decimal128 source value to cover both function names: the exact PostgreSQL result matches, while the rounded Float64 result does not.

Testing

• Extension builds and installs with -Werror
• Numeric power regression verified with PostgreSQL 19 and ClickHouse 26.7

[theory]

ClickHouse pow() takes a Decimal value, so we should be able to evaluate numerics without loss of precision.

[theory]

Oh I see, it returns Float64. I don't know whether this loss of precision matters, though.

[fallintoplace]

Yes, it matters for exact comparisons. In ClickHouse 26.7, pow(Decimal128(...), 2) accepts Decimal input but returns Float64, producing 1.5241578753238834 instead of PostgreSQL’s exact numeric result 1.524157875323883675. The regression demonstrates that pushing it down changes the result, so numeric pow()/power() should remain local while float8 remains pushed down.