Introduce scope-aware Context and #[MapContext]

[ruudk]

Resolvers often need per-request data that is neither a GraphQL input nor a DI service: the current HTTP request, the authenticated user, or a value computed by an ancestor field. Until now the generated resolver closures ignored webonyx's context value entirely, so the only escape hatches were #[Arg] (client input) and #[Autowire] (container), and there was no way to thread request state to a resolver.

This adds a first-class Context value object and a #[MapContext] parameter attribute. Context is a webonyx ScopedContext that stores objects by class (attachObject/getObject/maybeGetObject) and also implements ArrayAccess; because it is scoped, a value attached by a resolver is visible to its descendants without leaking to siblings. #[MapContext] injects an object from the Context into a resolver parameter by its type — nullable parameters resolve to null when absent, non-nullable parameters throw.

The generated query, mutation, and field closures now receive webonyx's context as a mixed value and pass it through to ArgumentNodeResolver. The value is left untouched (it can be anything the server provides); only #[MapContext] resolution requires it to be a Context, throwing a ContextException otherwise. Wiring a Context onto the server is up to the consumer via the webonyx context option (see the docs).