fix fresh-host acceptance and document real-host debugging learnings

tests/integration/host_stub_test.go

@@ -149,7 +149,7 @@ func TestSub2APIHostAdapterTestAccountParsesSSE(t *testing.T) {
 		t.Fatalf("NewClient() error = %v", err)
 	}
 
-	result, err := client.TestAccount(context.Background(), "account_1")
+	result, err := client.TestAccount(context.Background(), "account_1", "deepseek-chat")
 	if err != nil {
 		t.Fatalf("TestAccount() error = %v", err)
 	}
@@ -591,7 +591,7 @@ func newSub2APIStubServer(t *testing.T, cfg sub2APIStubConfig) *httptest.Server
 		})
 	})
 	mux.HandleFunc("/v1/models", func(w http.ResponseWriter, r *http.Request) {
-		if got := r.Header.Get("x-api-key"); got != gatewayExpectedKey {
+		if got := r.Header.Get("Authorization"); got != "Bearer "+gatewayExpectedKey {
 			w.WriteHeader(http.StatusUnauthorized)
 			_, _ = w.Write([]byte(`{"error":"unauthorized"}`))
 			return
@@ -600,6 +600,26 @@ func newSub2APIStubServer(t *testing.T, cfg sub2APIStubConfig) *httptest.Server
 			"data": gatewayModels,
 		})
 	})
+	mux.HandleFunc("/v1/chat/completions", func(w http.ResponseWriter, r *http.Request) {
+		if got := r.Header.Get("Authorization"); got != "Bearer "+gatewayExpectedKey {
+			w.WriteHeader(http.StatusUnauthorized)
+			_, _ = w.Write([]byte(`{"error":"unauthorized"}`))
+			return
+		}
+		writeJSON(t, w, http.StatusOK, map[string]any{
+			"id":     "chatcmpl_stub",
+			"object": "chat.completion",
+			"choices": []map[string]any{
+				{
+					"index": 0,
+					"message": map[string]any{
+						"role":    "assistant",
+						"content": "pong",
+					},
+				},
+			},
+		})
+	})
 
 	return httptest.NewServer(mux)
 }