fix(providers): add circuit breaker for Responses API fallback

When the Responses API fails repeatedly (3 consecutive compatibility
errors), skip it and fall back directly to Chat Completions.  Unlike a
permanent disable, the circuit re-probes after 5 minutes so recovery
is automatic when the API comes back.  Success resets the counter.

Keyed per (model, reasoning_effort) so a failure with one model does
not affect others.

tests/providers/test_responses_circuit_breaker.py

@@ -0,0 +1,71 @@
+"""Tests for Responses API circuit breaker in OpenAICompatProvider."""
+
+import time
+
+import pytest
+
+from nanobot.providers.openai_compat_provider import (
+    OpenAICompatProvider,
+    _RESPONSES_FAILURE_THRESHOLD,
+    _RESPONSES_PROBE_INTERVAL_S,
+)
+
+
+@pytest.fixture()
+def provider():
+    """A direct-OpenAI provider with Responses API support."""
+    p = OpenAICompatProvider.__new__(OpenAICompatProvider)
+    p.default_model = "gpt-5"
+    p._spec = type("Spec", (), {"name": "openai"})()
+    p._effective_base = "https://api.openai.com/v1"
+    p._responses_failures = {}
+    p._responses_tripped_at = {}
+    return p
+
+
+def test_responses_api_available_by_default(provider):
+    assert provider._should_use_responses_api("gpt-5", None) is True
+
+
+def test_circuit_opens_after_threshold(provider):
+    for _ in range(_RESPONSES_FAILURE_THRESHOLD):
+        provider._record_responses_failure("gpt-5", None)
+    assert provider._should_use_responses_api("gpt-5", None) is False
+
+
+def test_circuit_does_not_affect_other_models(provider):
+    for _ in range(_RESPONSES_FAILURE_THRESHOLD):
+        provider._record_responses_failure("gpt-5", None)
+    assert provider._should_use_responses_api("o4-mini", None) is True
+
+
+def test_success_resets_circuit(provider):
+    for _ in range(_RESPONSES_FAILURE_THRESHOLD):
+        provider._record_responses_failure("gpt-5", None)
+    assert provider._should_use_responses_api("gpt-5", None) is False
+    provider._record_responses_success("gpt-5", None)
+    assert provider._should_use_responses_api("gpt-5", None) is True
+
+
+def test_probe_after_interval(provider, monkeypatch):
+    for _ in range(_RESPONSES_FAILURE_THRESHOLD):
+        provider._record_responses_failure("gpt-5", None)
+    assert provider._should_use_responses_api("gpt-5", None) is False
+
+    # Fast-forward past the probe interval
+    key = "gpt-5:"
+    provider._responses_tripped_at[key] = time.monotonic() - _RESPONSES_PROBE_INTERVAL_S - 1
+    assert provider._should_use_responses_api("gpt-5", None) is True
+
+
+def test_below_threshold_still_allows(provider):
+    provider._record_responses_failure("gpt-5", None)
+    provider._record_responses_failure("gpt-5", None)
+    assert provider._should_use_responses_api("gpt-5", None) is True
+
+
+def test_reasoning_effort_keyed_separately(provider):
+    for _ in range(_RESPONSES_FAILURE_THRESHOLD):
+        provider._record_responses_failure("o3", "high")
+    assert provider._should_use_responses_api("o3", "high") is False
+    assert provider._should_use_responses_api("o3", "low") is True