feat: default two structured re-asks and sharper AIMD cut

Round 5 left three residual failure classes; ladder exhaustion (3.2%
of structured calls with a single re-ask) and 429 leakage (5.8%, AIMD
oscillating above the sustainable point) are addressable: re-ask
default goes 1 to 2 (conservative vs instructor's 3) and the AIMD cut
factor drops to 0.5.

tests/unit/test_config.py

@@ -148,6 +148,13 @@ class TestResilienceKeys:
 
 
 class TestAssemblyGuards:
+    def test_structured_retries_default_two(self):
+        # M2.5 迭代 4: 缺省重问 1→2(instructor 缺省 3 的保守版;P6 阶梯死亡 3.2% 实证)
+        s = GatewaySettings.from_env("LLM", env=_env())
+        assert s.structured_max_retries == 2
+        s2 = GatewaySettings.from_env("LLM", env=_env(PGW_STRUCTURED_MAX_RETRIES="0"))
+        assert s2.structured_max_retries == 0
+
     def test_cache_requires_namespace_and_ttl(self):
         env = _env(PGW_CACHE_BACKEND="memory")
         with pytest.raises(ValueError, match="NAMESPACE"):