Fix ambiguous string member serialization in generated contexts

[HoMiyabi]

Generated serializers write string members with WriteScalar, so a string such as "null" is emitted without quotes and becomes a null reference on deserialization. Emit WriteString instead, matching root strings, collection elements, and reflection-based serialization.

Add regression coverage for nullable and non-nullable string members, ambiguous scalar values, empty strings, actual null values, and disabling ambiguous scalar quoting.

[xoofx]

Reviewed the generator change and regression coverage. WriteString correctly preserves ambiguous CLR string values while retaining null handling and the opt-out behavior. All 675 tests passed locally. Added and pushed documentation for the quoting behavior; the documentation build passes (with an existing unrelated CurrentKey xref warning).