refactor(graphql): migrate internal marker decorators to auto decorators - #105

What

Migrates the GraphQL library's internal marker decorators from hand-written implementations to compiler internal auto dec declarations, and adopts the tspd gen-extern-signature generated accessors + decorator signature types.

Decorators converted to internal auto dec:

• @nullable
• @nullableElements
• @oneOf — was still using the .decorators-push hack (model.decorators.push(...) / .some(...))
• @inputType — was an internal-only useStateSet marker whose $inputType decorator fn was dead (never registered)

Why

These decorators just mark a type; they store no meaningful data and had no validation/transform logic. Previously the emitter located them via .decorators scanning or bespoke state helpers. auto dec lets the compiler synthesize the implementation and lets tspd generate strongly-typed is*/set* accessors, removing hand-written state plumbing.

Changes

• lib/{nullable,one-of,input-type}.tsp: internal auto dec declarations (added input-type.tsp, wired into main.tsp).
• Deleted src/lib/{nullable,one-of,input-type}.ts — replaced by generated generated-defs/TypeSpec.GraphQL.ts accessors (isNullable/setNullable, isOneOf/setOneOf, isInputType/setInputType, ...).
• All decorator impls typed with generated *Decorator signatures; $decorators uses satisfies TypeSpecGraphQLDecorators.
• Build wires up gen-extern-signature; added tspconfig.yaml opting into the auto-decorators experimental feature.
• Removed now-unused GraphQLKeys state entries.

Decorators intentionally left as-is

@compose, @operationFields, @schema, @graphqlInterface, @mutation/@query/@subscription, @specifiedBy — these carry validation / accumulation / transform logic that auto decorators can't express.

Note

This branch includes the merge of microsoft/main which brings the setAutoDecorator compiler API + tspd setter generation (microsoft#11247) that this refactor depends on. pinterest/feature/graphql was exactly 1 commit behind main (that PR), so the diff surfaces those changes too.

All 368 @typespec/graphql tests pass; lint + prettier clean.