Sentry context processor registration — register.simple_tag alternative

Getsentry cross-repo CI: PR CI runs against pinned sentry SHA from sentry-version, not the PR's sentry branch. Mypy plugin additions in sentry PR aren't available until pin bump. HttpRequest attrs assigned in code must be declared in tools/mypy_helpers/plugin.py::_adjust_http_request_members or mypy fails with attr-defined. Workflow: merge sentry PR → bump bot updates sentry-version → rebase getsentry PR. Workarounds: setattr(request, 'name', value) # noqa: B010 or # type: ignore[attr-defined].