Fix attach session race that drops live frames

[jhgaylor]

Summary

WSCommand.start() creates the _run_io background task before calling _wait_for_session_info() on attach. Both coroutines then iterate async for message in self.ws against the same WebSocket. The websockets library does not support concurrent reads on a single connection — messages get split between the two coroutines unpredictably, and _wait_for_session_info silently drops any binary frames it happens to consume while waiting for the session_info text frame.

The visible symptom: a client that calls sprite.attach_session(id) reliably receives the replayed scrollback but loses subsequent live stdout/stderr/exit frames.

Fix

Drain _wait_for_session_info to completion before scheduling the _run_io task. There is then only ever one reader on the WebSocket at a time, and live binary frames are delivered in full.

-        self._io_task = asyncio.create_task(self._run_io())
-        await asyncio.sleep(0)
-
-        if self._is_attach:
-            await self._wait_for_session_info()
+        if self._is_attach:
+            await self._wait_for_session_info()
+
+        self._io_task = asyncio.create_task(self._run_io())
+        await asyncio.sleep(0)

Reproduction

Against a live sprite running bash -c 'for i in $(seq 1 30); do echo tick=$i; sleep 1; done':

Before — second client opened via sprite.attach_session(id) sees the replay and then nothing:

[reattached] tick=1
[reattached] tick=2
[reattached] tick=3
(no live ticks ever arrive on the reattached connection)

After — same setup, second client sees replay AND live frames in lockstep with the original exec:

[reattached] tick=1
[reattached] tick=2
[reattached] tick=3
[reattached] tick=4
[reattached] tick=5
...

A raw-WebSocket probe (bypassing the SDK) confirmed the server has always been streaming live frames to attach connections — the bug was purely client-side.

Test plan

• All 24 existing tests in tests/ still pass (pytest tests/)
• Verified against a live sprite end-to-end with sprite.attach_session(id) from the patched source
• No new tests added — the existing suite has no WebSocket-level coverage and stubbing the websockets async iterator semantics would balloon scope; happy to add one if you'd like