Add negative test for API circuit breaker trip/short-circuit

The apicircuit package already wraps all external API calls (OpenAI TTS,
Gemini TTS, OpenAI image/DALL-E, and Gemini Nano Banana image generation)
with sony/gobreaker circuit breakers. This adds the missing resilience
regression test verifying the breaker opens after the configured number of
consecutive failures and short-circuits subsequent calls with
gobreaker.ErrOpenState without invoking the wrapped function.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 38 insertions, 0 deletions

diff --git a/internal/apicircuit/apicircuit_test.go b/internal/apicircuit/apicircuit_test.go
index 5f84c14..b6251a7 100644
--- a/internal/apicircuit/apicircuit_test.go
+++ b/internal/apicircuit/apicircuit_test.go
@@ -4,6 +4,8 @@ import (
 	"context"
 	"errors"
 	"testing"
+
+	"github.com/sony/gobreaker"
 )
 
 func TestOpenAITTS_Success(t *testing.T) {
@@ -16,6 +18,42 @@ func TestOpenAITTS_Success(t *testing.T) {
 	}
 }
 
+// TestBreakerOpensAndShortCircuits verifies the core resilience guarantee: after
+// breakerTripAfterConsecutiveFailures failed calls the breaker opens, and further
+// calls are short-circuited with gobreaker.ErrOpenState without invoking fn. This
+// is what prevents a degraded API from causing cascading load.
+func TestBreakerOpensAndShortCircuits(t *testing.T) {
+	t.Parallel()
+
+	// Use a dedicated breaker (not the package singletons) so this test stays
+	// isolated from the others while still exercising the shared trip policy.
+	cb := newBreaker("test-trip")
+	apiErr := errors.New("upstream failure")
+
+	// Drive enough consecutive failures to trip the breaker open.
+	for i := uint32(0); i < breakerTripAfterConsecutiveFailures; i++ {
+		_, err := runValue(cb, func() (string, error) {
+			return "", apiErr
+		})
+		if !errors.Is(err, apiErr) {
+			t.Fatalf("call %d: got %v, want upstream failure", i, err)
+		}
+	}
+
+	// The breaker must now be open and short-circuit without calling fn.
+	called := false
+	_, err := runValue(cb, func() (string, error) {
+		called = true
+		return "ok", nil
+	})
+	if !errors.Is(err, gobreaker.ErrOpenState) {
+		t.Fatalf("expected ErrOpenState once breaker is open, got %v", err)
+	}
+	if called {
+		t.Fatal("open breaker must short-circuit and not invoke fn")
+	}
+}
+
 func TestIsSuccessful_ContextCanceled(t *testing.T) {
 	t.Parallel()
 	if !isSuccessful(context.Canceled) {