fix(rpc): stream v2 chunk frames with backpressure and recover stale page cursors

Two fixes on top of the paged transport:

- Near-limit v2 framing no longer materializes the full base64 transport:
  chunk lines are generated lazily from a single serialization, the 64 MiB
  reassembly ceiling is enforced via Buffer.byteLength before any
  full-payload allocation, and RPC stdout writes drain with backpressure
  one physical line at a time. Peak RSS for a 63 MiB response drops
  ~686 MB -> ~521 MB; a rejected 80 MiB response drops ~507 MB -> ~259 MB
  (parity with the v1 path).

- get_messages_page errors now carry a machine-readable code
  (session_busy | stale_cursor). Both bundled clients' high-level
  getMessages() drains discard partial pages and fall back to the legacy
  snapshot on either code — previously a cursor invalidated by a
  background mutation (e.g. an appended bash message) threw instead of
  falling back. Direct page calls remain strict.

python/omp-rpc/tests/test_client.py

@@ -519,6 +519,34 @@ V2_MESSAGES_SERVER = textwrap.dedent(
                         "command": command_type,
                         "success": False,
                         "error": "Cannot page messages while the session is changing",
+                        "code": "session_busy",
+                    }
+                )
+                continue
+            if os.environ.get("V2_MESSAGES_STALE") == "1":
+                if command.get("cursor") is not None:
+                    emit(
+                        {
+                            "id": request_id,
+                            "type": "response",
+                            "command": command_type,
+                            "success": False,
+                            "error": "RPC message cursor is stale",
+                            "code": "stale_cursor",
+                        }
+                    )
+                    continue
+                emit(
+                    {
+                        "id": request_id,
+                        "type": "response",
+                        "command": command_type,
+                        "success": True,
+                        "data": {
+                            "messages": [message],
+                            "totalMessages": 2,
+                            "nextCursor": "page-two",
+                        },
                     }
                 )
                 continue
@@ -1036,6 +1064,17 @@ class RpcClientTests(unittest.TestCase):
         self.assertEqual(len(messages), 1)
         self.assertEqual(messages[0]["content"][0]["text"], "streaming snapshot")
 
+    def test_protocol_v2_get_messages_discards_stale_page_walk(self) -> None:
+        with self.make_client(
+            server=V2_MESSAGES_SERVER, env={"V2_MESSAGES_STALE": "1"}
+        ) as client:
+            with self.assertRaisesRegex(RpcCommandError, "RPC message cursor is stale"):
+                client.get_messages_page(cursor="page-two")
+            messages = client.get_messages()
+
+        self.assertEqual(len(messages), 1)
+        self.assertEqual(messages[0]["content"][0]["text"], "streaming snapshot")
+
     def test_collect_events_returns_turn_events(self) -> None:
         with self.make_client() as client:
             client.prompt("slow")