test(sdk-py): check that a reconnect loop resumes after the newest event (#9483)

Follow-up to the review on #9482: with `stream_resumable=True`, does a
loop of reconnects replay events? Each reconnect sends the newest event
id the stream got, and a failed reconnect keeps that id for the next
one, so the server replays only what the client missed. The existing
loop tests served the same events whatever the cursor, so nothing
checked this.

`test_*_reconnect_loop_resumes_after_the_newest_event` checks the
`Last-Event-ID` of every request in two loops, sync and async: progress
between drops, and a 502 between two reconnects. A stream that kept its
first cursor, or dropped it after a failed reconnect, fails it.

libs/sdk-py/tests/test_client_stream.py

@@ -798,6 +798,39 @@ async def test_async_long_stream_survives_many_spaced_out_drops():
     assert [p.data for p in parts] == [0, 1, 2, 3, 4, 5, 6, None]
 
 
+# Each reconnect resumes after the newest event the stream got, so a reconnect
+# loop on a resumable run replays only what it missed. A failed reconnect keeps
+# the cursor for the next one.
+RECONNECT_LOOPS = {
+    "progress_between_drops": (
+        lambda stream_cls: _long_stream_steps(stream_cls, drops=3),
+        [None, "0", "1", "2"],
+    ),
+    "failed_reconnect": (
+        lambda stream_cls: _transient_steps(stream_cls, _status(502)),
+        [None, "1", "1"],
+    ),
+}
+
+
+@pytest.mark.usefixtures("no_sleep")
+@pytest.mark.parametrize(
+    ("steps", "cursors"), RECONNECT_LOOPS.values(), ids=RECONNECT_LOOPS.keys()
+)
+def test_sync_reconnect_loop_resumes_after_the_newest_event(steps, cursors):
+    _, seen = _run_sync(steps(ListByteStream))
+    assert [r.headers.get("last-event-id") for r in seen] == cursors
+
+
+@pytest.mark.usefixtures("no_sleep")
+@pytest.mark.parametrize(
+    ("steps", "cursors"), RECONNECT_LOOPS.values(), ids=RECONNECT_LOOPS.keys()
+)
+async def test_async_reconnect_loop_resumes_after_the_newest_event(steps, cursors):
+    _, seen = await _run_async(steps(AsyncListByteStream))
+    assert [r.headers.get("last-event-id") for r in seen] == cursors
+
+
 # Reconnects should back off rather than hammer the server.