fix(desktop): rebind sessions after websocket reconnect (salvage of #41740) (#43004)

* fix(desktop): rebind sessions after websocket reconnect

* docs(desktop): explain the reconnect-resume guard in use-route-resume

The reconnect fix turns on two subtle conditions with no inline rationale:
`seenGatewayStateRef` suppresses a spurious "became open" on the first effect
run (so a session mounting with the gateway already open doesn't double-resume),
and the `gatewayBecameOpen ||` arm forces a re-resume even when the route looks
`alreadyActive` because the cached runtime id can be stale after the gateway
rebinds/reaps the session. Comment both so the next reader doesn't "simplify"
them back into the original bug. No behavior change.

---------

Co-authored-by: Josh Dow <josh.dow@prepad.io>

tests/tui_gateway/test_protocol.py

@@ -613,6 +613,44 @@ def test_session_resume_live_payload_uses_current_history_with_ancestors(server,
     ]
 
 
+def test_session_activate_rebinds_orphaned_ws_session_to_current_transport(server, monkeypatch):
+    """Reconnect + activate must reattach a parked live session before orphan reap."""
+
+    class _Transport:
+        def write(self, _obj):
+            return True
+
+    sid = "runtime01"
+    old_transport = server._stdio_transport
+    new_transport = _Transport()
+    server._sessions[sid] = {
+        "agent": types.SimpleNamespace(model="test/model"),
+        "created_at": 123.0,
+        "history": [],
+        "history_lock": threading.RLock(),
+        "last_active": 123.0,
+        "running": False,
+        "session_key": "20260409_010101_abc123",
+        "transport": old_transport,
+    }
+    monkeypatch.setattr(server, "current_transport", lambda: new_transport)
+    monkeypatch.setattr(server, "_get_db", lambda: None)
+    monkeypatch.setattr(
+        server,
+        "_session_info",
+        lambda _agent, _session=None: {"model": "test/model"},
+    )
+
+    resp = server.handle_request(
+        {"id": "activate", "method": "session.activate", "params": {"session_id": sid}}
+    )
+
+    assert "error" not in resp
+    assert resp["result"]["session_id"] == sid
+    assert server._sessions[sid]["transport"] is new_transport
+    assert not server._ws_session_is_orphaned(server._sessions[sid])
+
+
 def test_session_branch_persists_branched_from_marker(server, monkeypatch):
     """TUI /branch must persist a _branched_from marker so the branch stays
     visible in /resume and /sessions.