Remove the legacy graph-building backend

[jitu5]

Parent issue: #2265
Depends on: #2659, #2660, #2661, #2687

Summary

Remove the old live graph-construction path after all runtime graph reads are served by the inspection provider and parity is proven. Keep only the live-object functionality still required for previews.

Scope

• Delete the old pipeline traversal, graph-edge construction, registered-pipeline graph repositories, and legacy graph response builders.
• Remove any fallback that can still serve a second graph engine.
• Keep the smallest same-ID live metadata/preview index needed while previews require real objects.
• Move the plain node-ID-to-layer sorter to the shared layer service, remove the old GraphNode-based sorter, and delete CatalogRepository.layers_mapping when unused.
• Move the VSCode get_kedro_project_json_data path to the provider or a tested compatibility wrapper.
• Give NotebookVisualizer an inspection-compatible in-memory graph path. If Kedro cannot yet create a snapshot from in-memory Pipeline/catalog objects, track and land that prerequisite before attempting full legacy deletion.

Acceptance criteria

• The inspection provider is the only engine serving main and pipeline graph responses.
• Full, --params, lite, export, metadata, and run-status tests pass.
• Full-mode previews retain their existing behaviour.
• VSCode no longer depends on the legacy graph response builder and passes compatibility tests.
• NotebookVisualizer has a working in-memory replacement and passes its integration tests. If that prerequisite is unavailable, this ticket remains blocked rather than deleting its backend.
• Repository-wide searches show no production references to deleted code.