Add SDK support for the "Detailed" create/update/delete object on relation array field

[kheiner]

What's Changed

• Added NestedItemsInput<RawItem> to NestedPartial so that the API's "Detailed" { create, update, delete } object is supported
• RelationPrimaryKey<Item> resolves the items id type for delete entries with a fallback to string | number
• update entries switch id?: ... to id: ... when id is a key ^{⭐ 1}

Tested Scenarios

• Added coverage for createItem/createItems/updateItem/updateItems/updateItemsBatch for the NestedPartial describe blocks
• I switched three .toEqualTypeOf() assertions to use .branded.toEqualTypeOf() since the intersection output was difficult to read. Vitest's Docs say to "This helper comes at a performance cost and can cause the TypeScript compiler to 'give up' if used with excessively deep types. Use it sparingly and only when necessary." When I checked the performance speed I found no discernible difference between branded and unbranded.
• Played with it a bit on a live sandbox just to make sure the shapes worked (delete truly only accepts plain primary keys and not { id } objects

Review Notes / Questions / Concerns

• ^{⭐ 1}0eaf256 is for the devs who live dangerously and don't use id for the primary key. I don't know how to protect them from themselves, so whatever they chose to be the pk is optional and up to them to pass it along.
• The { create, update, delete } object passes for JSON. So if a dev accidentally passes the CRUD object to a field that is not relational and is typed to receive JSON, they still get a code 200 as a response. The API does the same thing, just calling it out as a new place to make a mistake you can already make in the API.
• minor felt right but I kept switching between patch and minor in my head.
• Issue title says "M2O and M2M" but m2o isn't supported in the API.

Checklist

Leave unchecked where not applicable

• Tests added/updated
• Documentation PR created in Fix Headings on Relational Data Guide docs#824 (not a content change, but the headings were wrong so it's related)
• OpenAPI updated
  • Local specs package (@directus/specs)
  • PR created in directus/openapi
• SDK (@directus/sdk) updated to reflect the changes
• Types (@directus/types) updated to reflect the changes
• GraphQL schema updated to reflect the changes
• System data (@directus/system-data) updated for changes to system collections/fields/relations
• Database migration added for schema/system changes
• Environment variables documented for new/changed config
• App translations added for new user-facing strings
• Security implications apply

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Fixes #25955