fix: raise ValueError when external_id is used for NDJSON annotation import

[sowndappan5]

Description

This PR fixes a client-side validation error when external_id is used inside a Label data container during NDJSON serialization (used for annotation/prediction imports).

Previously, if a developer created a Label using only external_id, the NDJSON serializer tried to build a DataRow with both id and global_key as None, causing Pydantic to throw a generic ValidationError: Must set either id or global_key.

Because the Labelbox annotation import (MAL) API does not natively support referencing data rows using externalId, we added a descriptive validation check that raises a helpful ValueError instructing developers to use global_key or resolve their external_id to a data_row_id before uploading.

Fixes #1941

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Document change (fix typo or modifying any markdown files, code comments or anything in the examples folder only)

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Note

Low Risk
Adds pre-serialization validation and clearer errors only; valid uid/global_key import paths are unchanged.

Overview
NDJSON serialization (used for MAL/prediction and similar imports) now fails fast when a Label’s data row is referenced only via external_id, instead of surfacing a vague Pydantic DataRow validation error.

In NDJsonConverter.serialize, each label is checked before NDLabel.from_common: if label.data is present but has neither uid nor global_key, a ValueError is raised telling callers to use global_key or resolve external_id to a data row id.

A unit test asserts that MALPredictionImport.create_from_objects with data={"external_id": "..."} raises that message.

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