front: fix SyntaxError in _counting_iter (bare try without except/finally)

The previous commit (be9950a5) left a try: with no except/finally in the SSE
throughput counter, breaking import of the front. Remove the stray try (the
BackgroundTask _release already closes the upstream response).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

codai/frontproxy/app.py

@@ -724,9 +724,7 @@ class FrontProxy:
         async def _counting_iter():
             import time as _t
             t0 = _t.monotonic(); ntok = 0; last = 0.0
-            try:
             async for raw in rp_resp.aiter_raw():
-                    if _meas:
                 ntok += raw.count(b"data:")
                 now = _t.monotonic()
                 if now - last >= 0.5:              # refresh ~2×/s, keep it cheap