Fix importer to update existing nodes when id provided (upsert behavior)

[TimurKady]

Motivation

• The importer attempted to always create objects even when an id was present in the import payload, causing validation errors like {'id': ['Segment (Who) with this ID already exists.']}.
• The goal is to perform upsert semantics per-level during import so existing rows are updated and new rows are created.

Description

• Changed TreeNodeImporter.import_tree to collect ids for each depth level and load existing objects with in_bulk to decide between update vs create.
• For rows matching existing objects the importer applies incoming fields (excluding internal tree fields _depth and _path and id) to the model instance, validates with full_clean() and calls save() to persist updates.
• New objects are still constructed, validated and written with bulk_create, and counters created and updated are maintained in the import result.
• Added a regression test TreeNodeImporterTests.test_importer_updates_existing_object_when_id_present to ensure importer updates an object when id is provided instead of attempting to re-create it.

Testing

• Ran the full test suite with python -m pytest -q, which completed successfully with 15 passed, 1 skipped (and 3 warnings).
• The added regression test verifies the upsert scenario and passes as part of the suite.

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Codex Task