…pi-codegen#2010)

PR oapi-codegen#1387 added an `$isExternalRef` branch to the strict-{,fiber-,iris-}
interface templates that strips the `<NameTag>Response` suffix when
embedding an external response ref. That made external refs to a
`components/responses/...` resolve to the bare schema name (`N400`)
instead of the strict envelope (`N400JSONResponse`).

The result: when spec A and spec B both generate strict-server and A
$refs B's response component, A's local envelope embeds
`N400JSONResponse` while A's external-ref envelope embeds
`externalRef0.N400`. The two struct shapes are no longer identical, so
cross-package response casts (the standard pattern for sharing error
shapes across services) stop compiling — the regression filed as
oapi-codegen#2010.

Investigation showed there is no smarter alternative: non-strict server
modes emit no top-level type for `components/responses/...`, only
`models: true` (gives the bare alias) and `strict-server: true` (gives
the `<Name>JSONResponse` envelope, which is also the only form that
carries a `Headers` field in the with-headers case) do.

Changes:

- Drop the `$isExternalRef` carve-out from the three strict-interface
  templates so external refs use the same `<Name>JSONResponse`
  embedding as internal refs.
- Update `internal/test/issues/issue-removed-external-ref` golden
  output to match.
- Update `internal/test/issues/issue-2113`'s common-package config to
  also generate `strict-server: true`. The fixture was relying on the
  PR oapi-codegen#1387 behavior; under the new policy the destination of a
  strict-server external ref must also generate a strict server, so
  `StandardErrorJSONResponse` is in scope.
- Add `internal/test/issues/issue-2010` regression fixture: two specs
  with strict-server, the second `$ref`s the first's
  `components/responses/400`, and the test exercises the cross-package
  cast that was broken.
- README: note the cross-spec strict-server requirement under the
  strict-server section.

The earlier two commits of oapi-codegen#1387 are kept: the `Schema.IsExternalRef`
helper, and the alias-vs-defined-type fix for content-schema external
refs (which is a genuinely independent bug fix — methods can't be
attached to non-local aliases).

BREAKING CHANGE: external `$ref` to a `components/responses/...` from a
strict-server target now requires the destination spec to also generate
`strict-server: true`. This restores cross-package response casting
that worked in v2.0.0 and earlier.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>