fix(authn): restore OIDC standalone module hygiene

Problem

task test runs an OIDC standalone-hygiene gate with GOWORK=off, but CI does not run the equivalent gate. The local gate fails because authn/oidc/fixture_test.go imports the root-only package github.com/stacklok/mecatl/internal/adapter/server.

With the workspace enabled, the import resolves and the ordinary OIDC CI tests pass. With GOWORK=off, an external consumer of the opt-in authn/oidc module cannot import root internal/..., so both go mod tidy and go test ./... fail.

Regression source

The import was introduced by commit b7ca4a6359e44b4f7418efc2cbf89dcdbfcf11f4 in #796:

• PR: https://github.com/stacklok/mecatl/pull/796
• Commit: https://github.com/stacklok/mecatl/commit/b7ca4a6359e44b4f7418efc2cbf89dcdbfcf11f4

Expected behavior

• cd authn/oidc && GOWORK=off go mod tidy succeeds.
• cd authn/oidc && GOWORK=off go test ./... succeeds.
• CI enforces the same OIDC standalone gate as local task test.

Suggested repair

Keep the OIDC-module test limited to OIDC construction/JWKS behavior. Move or retain the HTTP middleware 503 assertion in the root server test suite, where internal/adapter/server belongs. Do not add a root-module requirement or replace directive to the OIDC module.