fix(chat): de-duplicate the double turn execution at its source

The UI POSTs the SSE stream and, when nothing streams to the browser (iOS can't
read a fetch-stream body → the fetch throws in ~1s), falls back to the blocking
endpoint. But the server-side stream runs to completion regardless, so BOTH turns
executed — double-persisting the message and (once logging became guaranteed)
double-logging the hand.

Make a turn idempotent instead of chasing why the client bails: the first request
for a (session, message) owns it; a concurrent duplicate waits on the owner's
Event and reuses its reply rather than running a second full turn. respond and
respond_stream both claim/await; a finally always releases waiters. Short window
so a genuine later resend still runs fresh. Verified with a threaded race: two
simultaneous calls, body runs once, both get the same reply.

Also fixes the duplicate user-message persistence (the same double-execution) that
was polluting reconstructed history. 6 dedup tests + concurrency check; suite 232.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_hand_dedup.py

@@ -70,7 +70,7 @@ def test_parse_prompt_records_straddles():
 # --- hero stack auto-fill from the last logged stack ----------------------
 
 def test_hero_stack_filled_from_last_stack_log(poker, monkeypatch):
-    sid = poker.start_session(venue="Meadows", stakes="1/3", buy_in=400)
+    poker.start_session(venue="Meadows", stakes="1/3", buy_in=400)
     poker.log_stack(275)                                   # his last reported stack
     monkeypatch.setattr(poker, "parse_hand",
                         lambda *a, **k: {"game": "NLH", "hero_involved": True,
@@ -84,7 +84,7 @@ def test_hero_stack_filled_from_last_stack_log(poker, monkeypatch):
 
 
 def test_stated_stack_is_never_overridden(poker, monkeypatch):
-    sid = poker.start_session(venue="Meadows", buy_in=400)
+    poker.start_session(venue="Meadows", buy_in=400)
     poker.log_stack(275)
     monkeypatch.setattr(poker, "parse_hand",
                         lambda *a, **k: {"game": "NLH", "hero_involved": True,