Record source project when adding ExCS projects

[cocomarine]

Status

• Part of https://github.com/RaspberryPiFoundation/digital-editor-issues/issues/1724

Why not reuse remixed_from_id ?

• Although they are created by similar processes, ExCS projects added to editor-api are different from remixes.
• A lesson project copied from an ExCS source would have a non-nil remixed_from_id -> can't be teacher_project_ids with current logic in ability -> regression in teacher's ability such as teachers losing read/show_context on their students' remixes of that lesson etc.
• Api::Projects::RemixesController#index and RemixSelection both query Project.where(remixed_from_id: ), so the ExCS source project would start reporting lesson projects as its remixes, which is not correct.
• For these reasons, decided to go with new column specifically for recording source_project.

What's changed?

• Added source_project_id column
• Updated attributes_for_copy of lesson create.rb to add source project
  • Also cleared remix columns for above reasons (and for the unlikely situation where source projects have these columns populated)
• Added tests for asserting source_projects are copied during lesson copy and project remix.

[github-actions]

Test coverage

93.5% line coverage reported by SimpleCov.
Run: https://github.com/RaspberryPiFoundation/editor-api/actions/runs/33168605106

[Copilot]

Pull request overview

This PR introduces a dedicated source_project_id lineage field on Project to track Experience CS “source” projects separately from remix lineage, avoiding behavior/permission regressions tied to remixed_from_id.

Changes:

• Add projects.source_project_id with a self-referential FK (nullified on source deletion) and corresponding Project associations (source_project / derived_projects).
• Ensure lesson project creation records source_project_id and explicitly clears remix fields during ExCS-to-lesson copying.
• Add specs covering association behavior, lesson copy behavior, and remix behavior when a project already has a source_project.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
app/models/project.rb	Adds self-referential associations for source_project and derived_projects.
lib/concepts/lesson/operations/create.rb	Sets source_project_id for ExCS-based lesson project copies and clears remix fields.
db/migrate/20260828095502_add_source_project_to_projects.rb	Adds source_project reference to projects with FK nullify-on-delete.
db/schema.rb	Reflects the new column, index, and FK.
spec/models/project_spec.rb	Adds association coverage and verifies nullification when source project is destroyed.
spec/concepts/project/create_remix_spec.rb	Verifies remixes preserve source_project_id while remixed_from_id points to the immediate parent.
spec/concepts/lesson/create_spec.rb	Asserts lesson creation records source_project_id and does not inherit remix fields from the source.
spec/concepts/lesson/create_copy_spec.rb	Ensures lesson copy preserves source_project_id when copying a lesson whose project already has a source.

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