fix: reject non-finite floating-point default values

[huan233usc]

What

A float / double column default of NaN or ±Infinity passes SchemaField::Validate but cannot survive a metadata round-trip: the JSON serializer emits a non-finite number as null, which reads back as an absent default. The declared default is silently lost (and, because the key is present but null, the reader then errors on it).

How

ValidateDefault now rejects a non-finite floating default (std::isfinite on the float/double value) with a clear "value must be finite" error, so an unrepresentable default is caught up front rather than silently dropped on serialization.

Testing

SchemaFieldTest.ValidateRejectsNonFiniteFloatingDefault (NaN, +Inf, -Inf) and ValidateAcceptsFiniteFloatingDefault, verified fail-without / pass-with. Full schema_test passes (551 tests).