feat(security): apply audited PR #6738 remediations and test hardening

- Enforce vision probe SSRF safe local host validation, SafeProbeBackend TOCTOU elimination, and session owner authorization
- Authorize session titling endpoints against registered owner configuration
- Sanitize sensitive metadata logging from agent loop debug logs
- Add async non-blocking execution for spec router tools and thinking decisions
- Harden email cache invalidation fencing and thread-safe mutation
- Add regression test suites for legacy browser authority, spec contracts, spec router request scoping, and task MCP delivery

routes/research/research_routes.py

@@ -290,14 +290,9 @@ def setup_research_routes(research_handler, session_manager=None) -> APIRouter:
         # (most sessions have none), not an error. Return HTTP 200 with an
         # explicit empty status so loading a session does not log a spurious
         # 404; every client treats a non-"running" status as idle anyway.
-        # A session whose research exists but belongs to ANOTHER user stays a
-        # 404, so we do not leak the existence of someone else's research.
+        # Research owned by ANOTHER user answers identically, so the response
+        # does not reveal whether that research exists.
         if not _owns_in_memory(session_id, user):
-            exists = session_id in getattr(research_handler, "_active_tasks", {})
-            if not exists:
-                exists = _find_research_path(session_id) is not None
-            if exists:
-                raise HTTPException(404, "No research found for this session")
             return {"status": "none"}
         status = research_handler.get_status(session_id)
         if status is None: