fix: skip hedge dispatch when primary finishes during admission

Address branch review findings 1-4 on feature/1.3.7-hedged-requests:

- src/polygateway/middleware/retry.py: recheck primary.done() after
  hedge admission in _attempt_hedged; release the hedge permit via
  settle_and_release(permit, 0) (release_probe for probe entries) and
  adjudicate the primary directly instead of firing a billable HTTP
  request that would be cancelled immediately
- src/polygateway/config.py: check_hedge_assembly raises a hedge-located
  ValueError for empty sources instead of a bare min() error
- tests/unit/test_hedge.py: pin that an injected FakeClock jump of 10^6
  seconds does not trigger hedging (design section 8); pin pick(exclude)
  counting no gate_rejections and leaving reasons untouched; pin silent
  hedge abandonment when the candidate circuit is open; deterministic
  regression for the admission-window race (BlockingLimiter harness)
- tests/unit/test_config.py: assert the empty-sources guard message
  locates the hedge key

Red-to-green evidence in tests/outputs/137/review-fixes/

tests/unit/test_config.py

@@ -1166,6 +1166,31 @@ class TestHedgeConfig:
         with pytest.raises(ValueError, match=r"GatewayClient\(hedge_after_s"):
             _client(hedge_after_s=0)
 
+    def test_hedge_guard_empty_sources_raises_with_hedge_location(self):
+        """直传路空 sources + 设阈值: ValueError 且消息定位到 hedge(不是裸 min() 报错)。"""
+        from polygateway.config import check_hedge_assembly
+
+        with pytest.raises(ValueError, match="sources") as ei:
+            check_hedge_assembly(
+                hedge_after_s=8,
+                hedge_max_extra=1,
+                sources=[],
+                call_deadline_s=None,
+                origin="GatewayClient(hedge_after_s=8)",
+            )
+        assert "hedge_after_s" in str(ei.value)  # 定位得到是哪个键
+        # 未启用对冲(None)时空 sources 直接放行: 交叉守卫没有可校验的对象
+        assert (
+            check_hedge_assembly(
+                hedge_after_s=None,
+                hedge_max_extra=1,
+                sources=[],
+                call_deadline_s=None,
+                origin="test",
+            )
+            is None
+        )
+
     def test_hedge_guard_below_min_timeout_raises(self):
         """阈值 ≥ 最小源 timeout_s = 对冲永不可能触发,装配期炸掉(ValueError)。"""
         env = self._two_source_env(**{"LLM__HEDGE__AFTER_S": "90"})  # min(timeout)=90