fix(02): write fixtures exclusively and only on successful flush (WR-01)

Co-authored-by: Cursor <cursoragent@cursor.com>

tide/proxy_test.go

@@ -305,6 +305,40 @@ func TestProxyDuplicateSessionName(t *testing.T) {
 	}
 }
 
+func TestProxyFailedSessionLeavesNoFixture(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		if r.URL.Path == "/ok" {
+			_, _ = w.Write([]byte(`{"ok":true}`))
+			return
+		}
+		_, _ = w.Write([]byte(`{"token":"` + testJWT + `"}`))
+	}))
+	t.Cleanup(upstream.Close)
+	fixtures := t.TempDir()
+	proxy := newTestProxy(t, upstream.URL, fixtures, testRulesYAML())
+	srv := httptest.NewServer(proxy.Handler())
+	t.Cleanup(srv.Close)
+
+	doProxy(t, srv.URL, "partial", "/ok", "")
+	req, err := http.NewRequest(http.MethodGet, srv.URL+"/leak", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	req.Header.Set(SessionHeader, "partial")
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+	if err := proxy.Flush(); err == nil {
+		t.Fatal("failed session flush must surface the capture error")
+	}
+	if _, err := os.Stat(filepath.Join(fixtures, "nuxt", "partial.yaml")); !os.IsNotExist(err) {
+		t.Fatalf("partial session fixture committed: %v", err)
+	}
+}
+
 func newTestProxy(t *testing.T, upstream, fixtures, rulesYAML string) *Proxy {
 	t.Helper()
 	p, err := NewProxy(ProxyConfig{