fix(gateway): carry desktop_contract when activating a lazy session (#68392)

_live_session_payload() falls back to _fallback_session_info() while a
session's agent is still None (lazy/deferred build). That fallback omitted
desktop_contract, so session.activate returned lazy metadata with no contract
field. Desktop feeds the value straight into reportBackendContract(), where a
missing field reads as contract 0 — a current backend is then falsely flagged
"Backend out of date" on every activate of a live lazy session.

The sibling session.create shape (_lazy_resume_info) was fixed the same way in
#36112; this closes the remaining session.activate gap by advertising
DESKTOP_BACKEND_CONTRACT in the fallback payload.

Adds test_session_activate_lazy_info_reports_desktop_contract pinning the
session.activate path against a lazy (agent=None) session.

tests/test_tui_gateway_server.py

@@ -11270,6 +11270,41 @@ def test_session_create_lazy_info_reports_desktop_contract(monkeypatch):
     server._sessions.pop(resp["result"]["session_id"], None)
 
 
+def test_session_activate_lazy_info_reports_desktop_contract():
+    """Activating an already-live *lazy* session (agent not built yet) must
+    still advertise desktop_contract. _live_session_payload falls back to
+    _fallback_session_info while session["agent"] is None; the desktop reads a
+    missing field as contract 0 and falsely warns "Backend out of date" against
+    a current backend (#68392). The sibling session.create path was fixed in
+    #36112; this pins the session.activate path."""
+    import threading
+
+    sid = "lazy-activate-contract"
+    server._sessions[sid] = {
+        "agent": None,
+        "created_at": 123.0,
+        "history": [],
+        "history_lock": threading.RLock(),
+        "last_active": 123.0,
+        "running": False,
+        "session_key": sid,
+        "transport": server._stdio_transport,
+    }
+    try:
+        resp = server.handle_request(
+            {
+                "id": "activate-lazy",
+                "method": "session.activate",
+                "params": {"session_id": sid},
+            }
+        )
+        info = resp["result"]["info"]
+        assert info["lazy"] is True
+        assert info["desktop_contract"] == server.DESKTOP_BACKEND_CONTRACT
+    finally:
+        server._sessions.pop(sid, None)
+
+
 def test_session_list_returns_clean_error_when_state_db_is_unavailable(monkeypatch):
     monkeypatch.setattr(server, "_get_db", lambda: None)
     monkeypatch.setattr(server, "_db_error", "locking protocol")

tui_gateway/server.py

@@ -8083,6 +8083,12 @@ def _fallback_session_info(session: dict) -> dict:
         "model": _resolve_model(),
         "skills": {},
         "tools": {},
+        # A lazy session (agent not built yet) is still served by *this* backend,
+        # so it must advertise the current contract. Desktop feeds this straight
+        # into reportBackendContract(); a missing field is read as contract 0 and
+        # a current backend is falsely flagged "out of date" (#68392). The sibling
+        # session.create shape (_lazy_resume_info) already carries it (#36112).
+        "desktop_contract": DESKTOP_BACKEND_CONTRACT,
     }