Function objectToTableItem

Recursively walks an ObjectSchema and converts the entity value to its DynamoDB representation.

• "date" fields are converted from Date objects to ISO 8601 strings.
• "object" fields recurse into their nested schema.
• "array" fields map each item through the same conversion.
• "discriminatedUnion" fields look up the variant schema by discriminator value and recurse into that variant's object schema, preserving the discriminator key.
• null and undefined values are stripped from the result so that nullable fields set to null are removed from the stored object rather than persisted as null in DynamoDB.
• All other field types pass through unchanged.