fix(firestore-bigquery-change-tracker): keep partition value on delete using old data

[CorieW]

Resolves #2303

• Tested

[Copilot]

Pull Request Overview

This PR updates the partitioning logic to retain the partition value on delete events by sourcing it from the old data rather than the new event data.

• Uses the new ChangeType import to differentiate between DELETE and non-DELETE events.
• Applies a conditional to select the appropriate field value for partitioning.

Comments suppressed due to low confidence (1)

firestore-bigquery-export/firestore-bigquery-change-tracker/src/bigquery/partitioning.ts:202

• [nitpick] Consider adding an inline comment explaining why event.oldData is used for DELETE events. This will improve clarity and maintainability for future developers.

event.operation === ChangeType.DELETE ? event.oldData[firestoreFieldName] : event.data[firestoreFieldName];