Remove flowchart models unused after legacy graph removal

[jitu5]

Parent issue: #2265
Depends on: #2724

Summary

After the legacy graph builder is removed, delete only the internal flowchart models, repositories, and helpers that have no remaining production caller. This is cleanup only; API responses must not change.

Candidate removals

• the domain GraphEdge and its retired repository;
• RegisteredPipeline and its retired repository;
• GraphNode.belongs_to_pipeline();
• Tag, NamedEntity, and TagsRepository, if repository-wide searches show no caller remains.

Verify every candidate after the legacy deletion lands. Keep GraphNode wrappers, kedro_obj, metadata models, and preview helpers for as long as the full-mode preview path or another supported consumer still needs them.

Acceptance criteria

• No production reference remains to any removed symbol.
• /api/main, /api/pipelines/{id}, /api/nodes/{id}, run status, and export responses are unchanged.
• Full-mode previews and supported notebook/VSCode integrations remain functional.
• Ruff, mypy, and the full test suite pass.