review-fix(public-api): restore get_session_activity, latest_user_message_row_id, resolve_multiple_toolsets, has_provider, nous_token_has_billing_scope, curated_models_for_provider, clear_edit_approval_requester + tests

All public on BASE 63279301bc, dropped by the simplify refactor (their tests were deleted or
rewritten to the replacement API). Restore each with BASE signature/body as a thin wrapper over the
surviving implementation, and restore the tests at the original call sites: test_message_reactions
again asserts the role=user contract (a newer assistant message is never the default target);
test_hermes_state / test_watchdog_review_76354 go back to get_session_activity(); toolsets, acp auth,
edit_approval, billing-scope and curated-models tests restored/extended.

hermes_state_sessions.py

@@ -538,6 +538,21 @@ class SessionSessionsMixin:
             ("", ActivityProvenance.UNKNOWN.value, session_id), patience_s=self._ACTIVITY_WRITE_PATIENCE_S,
         )
 
+    def get_session_activity(self, session_id: str) -> Optional[Dict[str, Any]]:
+        """Return the durable activity snapshot for *session_id*, or None."""
+        if not session_id:
+            return None
+        row = self.get_session(session_id)
+        if not row:
+            return None
+        from agent.session_activity import build_activity_snapshot
+
+        return build_activity_snapshot(
+            last_activity_at=row.get("last_activity_at"),
+            last_activity_description=row.get("last_activity_description"),
+            last_activity_provenance=row.get("last_activity_provenance"),
+        )
+
     def update_session_meta(
         self, session_id: str, model_config_json: str, model: Optional[str] = None,
     ) -> None: