Adds federation error code coerage docs

[PascalSenn]

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[linux-foundation-easycla]

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• ✅ login: PascalSenn (5f9ee4d, e4f8774)

[kamilkisiela]

extend schema
              @link(
                url: "https://specs.apollo.dev/federation/v2.3"
                import: ["@key", "@requires"]
              )

            type Query {
              users: [User]
            }

            type User @key(fields: "id") {
              id: ID!
              internalId: ID!
              profile: Profile @requires(fields: "internalId")
            }

            type Profile {
              name: String
            }

[kamilkisiela]

[users] On field "User.profile", for @requires(fields: "internalId"): field "User.internalId" should not be part of a @requires since it is already provided by this subgraph (it is not marked @external)

[kamilkisiela]

extend schema
              @link(
                url: "https://specs.apollo.dev/federation/v2.3"
                import: ["@key", "@requires"]
              )

            type Query {
              users: [User]
            }

            type RegisteredUser implements User @key(fields: "id") {
              id: ID!
              name: String!
              email: String
            }

            interface User @key(fields: "id") {
              id: ID!
              name: String!
              email: String @requires(fields: "name")
            }

[users] Cannot use @requires on field "User.email" of parent type "User": @requires is not yet supported within interfaces

[kamilkisiela]

# subgraph book

extend schema @link(url: "https://specs.apollo.dev/federation/v2.3", import: ["@key"])

            interface Media @key(fields: "id") {
              id: ID!
              title: String!
            }

            type Book implements Media @key(fields: "id") {
              id: ID!
              title: String!
            }
            
# subgraph review

extend schema
              @link(
                url: "https://specs.apollo.dev/federation/v2.3"
                import: ["@key", "@interfaceObject"]
              )

            type Media @interfaceObject {
              id: ID!
              reviews: [Review!]!
            }

            type Review {
              score: Int!
            }

            type Query {
              topRatedMedia: [Media!]!
            }

The @interfaceObject directive can only be applied to entity types but type "Media" has no @key in this subgraph.

[kamilkisiela]

or

# subgraph a
extend schema
                  @link(
                    url: "https://specs.apollo.dev/federation/v2.3"
                    import: ["@key", "@interfaceObject"]
                  )

                type Query {
                  hello: String
                }

# subgraph b
extend schema
                  @link(
                    url: "https://specs.apollo.dev/federation/v2.3"
                    import: ["@key", "@interfaceObject"]
                  )
                type Query {
                  otherField: String
                }

                type MyInterface @key(fields: "id") @interfaceObject {
                  id: ID!
                  newField: String
                }                
                ```
                

Type "MyInterface" is declared with @interfaceObject in all the subgraphs in which is is defined (it is defined in subgraph "b" but should be defined as an interface in at least one subgraph)

[kamilkisiela]

extend schema @link(url: "https://specs.apollo.dev/federation/v2.3", import: ["@key"])

            type Query {
              users: [User!]!
            }

            type User @key(fields: true) {
              id: ID
            }

[users] On type "User", for @key(fields: true): Invalid value for argument "fields": must be a string.

[kamilkisiela]

There are a few examples, I can provide them if needed, but those are relevant errors messages:

[users] Cannot apply merged directive @inaccessible to external field "User.internalId"

[users] Cannot apply merged directive @tag(name: "public") to external field "User.internalId"

[kamilkisiela]

extend schema
              @link(
                url: "https://specs.apollo.dev/federation/v2.3"
                import: ["@key", "@provides"]
              )

            type Payment @key(fields: "id") {
              id: ID!
              amount: Int!
            }

            type Invoice @key(fields: "id") {
              id: ID!
              amount: Int!
              payment: Payment @provides(fields: true)
            }

[billing] On field "Invoice.payment", for @provides(fields: true): Invalid value for argument "fields": must be a string.

[kamilkisiela]

extend schema
              @link(
                url: "https://specs.apollo.dev/federation/v2.3"
                import: ["@key", "@provides"]
              )

            type Query {
              users: [User]
            }

            type RegisteredUser implements User @key(fields: "id") {
              id: ID!
              name: String!
              email: String
            }

            interface User @key(fields: "id") {
              id: ID!
              name: String!
              email: String @provides(fields: "name")
            }

[users] Invalid @provides directive on field "User.email": field has type "String" which is not a Composite Type

[kamilkisiela]

extend schema
              @link(
                url: "https://specs.apollo.dev/federation/v2.1"
                import: ["@key", "@provides"]
              )

            type Query {
              users: [User]
            }

            type RegisteredUser implements User @key(fields: "id") {
              id: ID!
              name: String!
              profile: Profile
            }

            interface User @key(fields: "id") {
              id: ID!
              name: String!
              profile: Profile @provides(fields: "name")
            }

            type Profile {
              name: String
            }

[users] Cannot use @provides on field "User.profile" of parent type "User": @provides is not yet supported within interfaces

[kamilkisiela]

I can provide more examples and cover more rules, just say so :)