[rubenvdlinde]

Part of #3099, on top of #3136 (menu saving). A menu item can sit under another item, one level deep, for example a few views under the table they belong to. This replaces #3088 by @remko48.

Size: 248 lines: 100 code, 128 tests and 20 generated OpenAPI.

What changes

• When saving a menu, an item can name its parent by its position in the list (parentIndex). Nesting goes one level deep: a parent can't have a parent of its own.
• The response gives each item its parentId.

How it was tested

On Nextcloud 34 with PostgreSQL:

• MenuItemInputTest (40 tests), ContextServiceTest (50 tests) and ContextControllerTest
• the full unit suite on the top of the stack (feat(contexts): carry the menu through export and import #3138), on PostgreSQL and SQLite: 665 tests
• composer cs:check, psalm and rector, all clean

🖼️ Screenshots

Backend only, no UI changes.

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔙 Backport requests are created or not needed: /backport to stableX.X
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI