refactor: extract magic number 0L to constant for new entity check

[currenjin]

Motivation

Replaced the hardcoded value 0L with a named constant to clarify its meaning as a marker for new (unsaved) entities.

Even though the value was only used in a single location, extracting it improves code readability and prevents potential misuse.

Changes

• Introduced a private static final constant NEW_ID = 0L
• Replaced inline 0L usage in new entity check

Checklists

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• You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.
• You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only).