feat(adapters): 实现 CircuitBreaker 内存级熔断器

注入 now 纯确定性，force_open 支持 401/403 直接熔断。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_breaker.py

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+"""CircuitBreaker 单元测试。
+
+验证内存级熔断器的核心行为：阈值触发、冷却半开、成功重置、强制熔断。
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from adapters.breaker import CircuitBreaker
+
+
+@pytest.fixture()
+def breaker() -> CircuitBreaker:
+    """创建 threshold=3, cooldown=10s 的熔断器。"""
+    return CircuitBreaker(fail_threshold=3, cooldown_s=10.0)
+
+
+class TestCircuitBreaker:
+    """CircuitBreaker 行为验证。"""
+
+    def test_closed_by_default(self, breaker: CircuitBreaker) -> None:
+        """初始状态为关闭（放行）。"""
+        assert breaker.is_open("llm", now=0.0) is False
+
+    def test_opens_after_threshold_failures(self, breaker: CircuitBreaker) -> None:
+        """连续失败未达阈值不开路；达到阈值后开路。"""
+        # 2 次失败 < threshold=3，仍关闭
+        breaker.record_failure("llm", now=1.0)
+        breaker.record_failure("llm", now=2.0)
+        assert breaker.is_open("llm", now=2.5) is False
+
+        # 第 3 次失败 >= threshold=3，开路
+        breaker.record_failure("llm", now=3.0)
+        assert breaker.is_open("llm", now=3.5) is True
+
+    def test_half_open_after_cooldown(self, breaker: CircuitBreaker) -> None:
+        """冷却期过后自动半开（放行试探）。"""
+        for t in range(3):
+            breaker.record_failure("llm", now=float(t))
+        # 开路期间
+        assert breaker.is_open("llm", now=5.0) is True
+        # cooldown=10s，第 3 次失败 now=2.0，open_until=12.0
+        # now=12.0 时冷却到期 → 半开
+        assert breaker.is_open("llm", now=12.0) is False
+
+    def test_success_resets(self, breaker: CircuitBreaker) -> None:
+        """record_success 重置计数并关闭熔断器。"""
+        for t in range(3):
+            breaker.record_failure("llm", now=float(t))
+        assert breaker.is_open("llm", now=3.0) is True
+
+        breaker.record_success("llm")
+        assert breaker.is_open("llm", now=3.0) is False
+
+    def test_force_open_immediate(self, breaker: CircuitBreaker) -> None:
+        """force_open 无需积累失败即可直接熔断。"""
+        breaker.force_open("llm", now=5.0)
+        assert breaker.is_open("llm", now=5.5) is True
+
+    def test_force_open_probe_failure_reopens(self, breaker: CircuitBreaker) -> None:
+        """半开探针失败后重新熔断（因 fails 已置为 threshold）。"""
+        breaker.force_open("llm", now=0.0)
+        # cooldown=10s → open_until=10.0，now=10.0 半开
+        assert breaker.is_open("llm", now=10.0) is False
+
+        # 半开探针失败：一次 record_failure 即达阈值（fails 已=3），重新熔断
+        breaker.record_failure("llm", now=10.0)
+        assert breaker.is_open("llm", now=10.5) is True