refactor: sweep the platform-and-sign-in epic

Split load_config into per-section readers, use contextlib.suppress and a single condition where ruff asked, remove stale noqa comments, add the SIM and RUF100 rules, share the sign_in test helper, align AD-6 with the code and record the remaining deferred findings.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N3t4WL7bd9bdHHNjEsxzqC

portal/tests/test_signin.py

@@ -14,20 +14,11 @@ from portal.accounts.session_store import LOGIN_FLOW, ROLES
 from portal.accounts.sessions import SessionUser
 from portal.adapters.idp import Flow
 from portal.audit.models import AuditEntry
+from portal.tests.helpers import sign_in
 
 pytestmark = pytest.mark.django_db(transaction=True)
 
 
-def sign_in(adapters, username="alice", next_path=None, client=None):
-    """Run the whole flow with the fake provider; returns (client, callback response)."""
-    client = client or Client()
-    adapters.idp.sign_in_as(username)
-    query = f"?next={next_path}" if next_path else ""
-    client.get("/auth/login" + query)
-    flow = Flow.from_dict(client.session[LOGIN_FLOW])
-    return client, client.get("/auth/callback", adapters.idp.callback_params(flow))
-
-
 def age_the_cache(client):
     session = client.session
     session[ROLES] = {**session[ROLES], "checked": time.time() - 3600}