fix: honor account model mapping before group fallback

backend/internal/service/openai_model_mapping_test.go

@@ -0,0 +1,70 @@
+package service
+
+import "testing"
+
+func TestResolveOpenAIForwardModel(t *testing.T) {
+	tests := []struct {
+		name               string
+		account            *Account
+		requestedModel     string
+		defaultMappedModel string
+		expectedModel      string
+	}{
+		{
+			name: "falls back to group default when account has no mapping",
+			account: &Account{
+				Credentials: map[string]any{},
+			},
+			requestedModel:     "gpt-5.4",
+			defaultMappedModel: "gpt-4o-mini",
+			expectedModel:      "gpt-4o-mini",
+		},
+		{
+			name: "preserves exact passthrough mapping instead of group default",
+			account: &Account{
+				Credentials: map[string]any{
+					"model_mapping": map[string]any{
+						"gpt-5.4": "gpt-5.4",
+					},
+				},
+			},
+			requestedModel:     "gpt-5.4",
+			defaultMappedModel: "gpt-4o-mini",
+			expectedModel:      "gpt-5.4",
+		},
+		{
+			name: "preserves wildcard passthrough mapping instead of group default",
+			account: &Account{
+				Credentials: map[string]any{
+					"model_mapping": map[string]any{
+						"gpt-*": "gpt-5.4",
+					},
+				},
+			},
+			requestedModel:     "gpt-5.4",
+			defaultMappedModel: "gpt-4o-mini",
+			expectedModel:      "gpt-5.4",
+		},
+		{
+			name: "uses account remap when explicit target differs",
+			account: &Account{
+				Credentials: map[string]any{
+					"model_mapping": map[string]any{
+						"gpt-5": "gpt-5.4",
+					},
+				},
+			},
+			requestedModel:     "gpt-5",
+			defaultMappedModel: "gpt-4o-mini",
+			expectedModel:      "gpt-5.4",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := resolveOpenAIForwardModel(tt.account, tt.requestedModel, tt.defaultMappedModel); got != tt.expectedModel {
+				t.Fatalf("resolveOpenAIForwardModel(...) = %q, want %q", got, tt.expectedModel)
+			}
+		})
+	}
+}