fix(openai): handle codex spark model limitations

backend/internal/service/openai_model_mapping_test.go

@@ -15,10 +15,19 @@ func TestResolveOpenAIForwardModel(t *testing.T) {
 			account: &Account{
 				Credentials: map[string]any{},
 			},
-			requestedModel:     "gpt-5.4",
+			requestedModel:     "claude-opus-4-6",
 			defaultMappedModel: "gpt-4o-mini",
 			expectedModel:      "gpt-4o-mini",
 		},
+		{
+			name: "preserves explicit gpt-5.4 instead of group default",
+			account: &Account{
+				Credentials: map[string]any{},
+			},
+			requestedModel:     "gpt-5.4",
+			defaultMappedModel: "gpt-4o-mini",
+			expectedModel:      "gpt-5.4",
+		},
 		{
 			name: "preserves exact passthrough mapping instead of group default",
 			account: &Account{
@@ -58,6 +67,42 @@ func TestResolveOpenAIForwardModel(t *testing.T) {
 			defaultMappedModel: "gpt-4o-mini",
 			expectedModel:      "gpt-5.4",
 		},
+		{
+			name: "preserves codex spark instead of group default",
+			account: &Account{
+				Credentials: map[string]any{},
+			},
+			requestedModel:     "gpt-5.3-codex-spark",
+			defaultMappedModel: "gpt-5.4",
+			expectedModel:      "gpt-5.3-codex-spark",
+		},
+		{
+			name: "preserves gpt-5.5 instead of group default",
+			account: &Account{
+				Credentials: map[string]any{},
+			},
+			requestedModel:     "gpt-5.5",
+			defaultMappedModel: "gpt-5.4",
+			expectedModel:      "gpt-5.5",
+		},
+		{
+			name: "preserves openai namespaced gpt-5.5 instead of group default",
+			account: &Account{
+				Credentials: map[string]any{},
+			},
+			requestedModel:     "openai/gpt-5.5",
+			defaultMappedModel: "gpt-5.4",
+			expectedModel:      "openai/gpt-5.5",
+		},
+		{
+			name: "preserves compact gpt-5.5 instead of group default",
+			account: &Account{
+				Credentials: map[string]any{},
+			},
+			requestedModel:     "gpt-5.5-openai-compact",
+			defaultMappedModel: "gpt-5.4",
+			expectedModel:      "gpt-5.5-openai-compact",
+		},
 	}
 
 	for _, tt := range tests {