feat(factory): add 99designs/gqlgen contrib with a boost-error ErrorPresenter (GraphQL = 4th transport) #44

Problem

boost maps its typed-error catalog to a transport code automatically on three edges:

• Echo error_handler → HTTP status
• gRPC server → codes.Code
• function / CloudEvents → deadletter / status

GraphQL is the gap. With 99designs/gqlgen, a resolver error is caught by gqlgen, run through its ErrorPresenter, and written as HTTP 200 with the error inside the errors[] array. The Echo error_handler never sees it, so the public extensions.code is not derived from the boost catalog — every service hand-rolls its own presenter.

Proposal

Ship a factory/contrib/99designs/gqlgen/v0 package providing a ready boost-aware ErrorPresenter (and optionally a thin server constructor mirroring the other factories) that:

1. Walks the stdlib error chain (boost typed errors now implement Unwrap()).
2. Maps the boost catalog → a stable extensions.code via the same classification the other transports use:
   • once the error registry (Classify / Kind, branch feat/error-registry) lands → a Kind → code table, sibling to restresponse.HTTPStatusFor and the gRPC Kind → codes.Code table;
   • meanwhile → the Is* catalog: IsNotFound→NOT_FOUND, IsServiceUnavailable→UPSTREAM_UNAVAILABLE, IsForbidden/IsConflict→FORBIDDEN, IsBadRequest/IsNotValid→BAD_USER_INPUT, default INTERNAL_SERVER_ERROR.
3. Never leaks internals for Internal / ServiceUnavailable (safe message).
4. Honors IgnoreOf (registry) the same way the other edges do.

This makes GraphQL the 4th transport over the single boost error catalog — srv.SetErrorPresenter(gqlgenfact.ErrorPresenter) and resolvers keep returning boost typed errors.

Notes / open questions

• Dependency surface: adds github.com/99designs/gqlgen + github.com/vektah/gqlparser/v2. Suggest isolating it as its own contrib module (own go.mod) so boost core doesn't take the dep, consistent with the other factory/contrib/*.
• The code-string set should be exported + overridable (each service maps to its own public GraphQL contract).
• Reference pattern is already documented in the boost-model-errors skill → section "GraphQL transport — gqlgen ErrorPresenter".
• Related: Err.Unwrap() (stdlib errors.Is/As transparency) already landed on main.

Acceptance

• factory/contrib/99designs/gqlgen/v0 with ErrorPresenter (+ tests) mapping the boost catalog → extensions.code
• Honors the error registry Classify / IgnoreOf once available
• Exported, overridable code strings
• Docs/skill (boost-model-errors and/or a new boost-factory-gqlgen) updated

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