fix: validate decimal default fits the field precision

[huan233usc]

What

SchemaField::Validate accepts a decimal default whose unscaled value does not fit the field's declared precision, even though the JSON reader rejects exactly that value on read-back — so a validated field can serialize to metadata that the same reader then refuses to parse.

Example: a decimal(2, 1) field with an initial-default of Literal::Decimal(999, 2, 1) (99.9, which needs 3 significant digits). ValidateDefault only checked that the literal's type matches the field type (decimal(2,1) == decimal(2,1)), so it passed; the value was serialized to "99.9"; on read-back Decimal::FromString yields precision 3 > 2 and the decimal literal parser (expression/json_serde.cc, and Literal::CastTo) rejects it. The field's own metadata becomes unreadable.

How

ValidateDefault now checks Decimal::FitsInPrecision(field precision) for decimal defaults after the type-equality check, rejecting an out-of-range value up front so it is consistent with what the reader accepts. This mirrors the same guard already applied when casting defaults during schema evolution.

Testing

SchemaFieldTest.ValidateRejectsDecimalDefaultExceedingPrecision (rejects 99.9 for decimal(2,1)) and ValidateAcceptsDecimalDefaultWithinPrecision (accepts a fitting value), verified fail-without / pass-with. Full schema_test passes (551 tests).