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 :)

[glen-84]

Suggested change

	This rule enforces that, for any subgraph schema, if a root mutation type is
	This rule enforces that, for any source schema, if a root mutation type is

[glen-84]

This example is unnecessarily verbose. There's no need for an object type here.

[glen-84]

Suggested change

	Valid Example:
	Valid example:

[glen-84]

Suggested change

	This rule enforces that the root query type in any subgraph schema must be named
	This rule enforces that the root query type in any source schema must be named

[glen-84]

Suggested change

	Valid Example:
	Valid example:

[glen-84]

Suggested change

	This rule enforces that, for any subgraph schema, if a root subscription type is
	This rule enforces that, for any source schema, if a root subscription type is

[glen-84]

Suggested change

	Valid Example:
	Valid example:

[glen-84]

Suggested change

# Subgraph A

[glen-84]

This example is unnecessarily verbose. There's no need for an object type here.

[glen-84]

Suggested change

# Subgraph A

[glen-84]

There can be multiple key directives.

[glen-84]

Or interface types.

[glen-84]

Suggested change

	In this following counter example, the `@key` directive references a field
	In this counter example, the `@key` directive references a field