Function convertFieldToTableItem

Converts a single field value to its DynamoDB table representation based on the field definition.

• "date" → ISO 8601 string
• "object" → recursively converts via objectToTableItem
• "array" → maps each item through the same conversion
• "discriminatedUnion" → looks up the variant schema by discriminator value, converts via objectToTableItem, and preserves the discriminator key
• All other types pass through unchanged