fix(openai): OAuth 透传路径补齐 input 规范化，修复 Input must be a list

openai_passthrough=true 的 OAuth 账号透传路径
(normalizeOpenAIPassthroughOAuthBody) 未对 input 字段做
string→array 转换，导致上游拒绝 "Input must be a list"。

正常 OAuth 路径 (openai_codex_transform.go:290) 已有此逻辑，
透传路径遗漏。补齐三种非数组 input 的规范化：
- string → [{type:"message",role:"user",content:<text>}]
- 空白 string → []
- 单 object → [<object>]

backend/internal/service/openai_gateway_request_body.go

@@ -575,6 +575,34 @@ func normalizeOpenAIPassthroughOAuthBody(body []byte, compact bool) ([]byte, boo
 		changed = true
 	}
 
+	if inputResult := gjson.GetBytes(normalized, "input"); inputResult.Exists() {
+		switch {
+		case inputResult.Type == gjson.String:
+			text := inputResult.String()
+			var inputValue any
+			if strings.TrimSpace(text) != "" {
+				inputValue = []map[string]any{{
+					"type": "message", "role": "user", "content": text,
+				}}
+			} else {
+				inputValue = []any{}
+			}
+			next, err := sjson.SetBytes(normalized, "input", inputValue)
+			if err != nil {
+				return body, false, fmt.Errorf("normalize passthrough body input string: %w", err)
+			}
+			normalized = next
+			changed = true
+		case inputResult.Type == gjson.JSON && !inputResult.IsArray():
+			next, err := sjson.SetRawBytes(normalized, "input", []byte("["+inputResult.Raw+"]"))
+			if err != nil {
+				return body, false, fmt.Errorf("normalize passthrough body input object: %w", err)
+			}
+			normalized = next
+			changed = true
+		}
+	}
+
 	if compact {
 		if store := gjson.GetBytes(normalized, "store"); store.Exists() {
 			next, err := sjson.DeleteBytes(normalized, "store")

backend/internal/service/openai_passthrough_normalization_test.go

@@ -31,3 +31,57 @@ func TestNormalizeOpenAIPassthroughOAuthBody_CompactRemovesUnsupportedUser(t *te
 	require.False(t, gjson.GetBytes(normalized, "stream").Exists())
 	require.False(t, gjson.GetBytes(normalized, "store").Exists())
 }
+
+func TestNormalizeOpenAIPassthroughOAuthBody_StringInputWrappedAsArray(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","input":"hello world"}`)
+
+	normalized, changed, err := normalizeOpenAIPassthroughOAuthBody(body, false)
+	require.NoError(t, err)
+	require.True(t, changed)
+
+	input := gjson.GetBytes(normalized, "input")
+	require.True(t, input.IsArray(), "string input should be converted to array")
+	items := input.Array()
+	require.Len(t, items, 1)
+	require.Equal(t, "message", items[0].Get("type").String())
+	require.Equal(t, "user", items[0].Get("role").String())
+	require.Equal(t, "hello world", items[0].Get("content").String())
+}
+
+func TestNormalizeOpenAIPassthroughOAuthBody_EmptyStringInputWrappedAsEmptyArray(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","input":"  "}`)
+
+	normalized, changed, err := normalizeOpenAIPassthroughOAuthBody(body, false)
+	require.NoError(t, err)
+	require.True(t, changed)
+
+	input := gjson.GetBytes(normalized, "input")
+	require.True(t, input.IsArray())
+	require.Len(t, input.Array(), 0, "whitespace-only input should become empty array")
+}
+
+func TestNormalizeOpenAIPassthroughOAuthBody_ObjectInputWrappedAsArray(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","input":{"type":"message","role":"user","content":"hi"}}`)
+
+	normalized, changed, err := normalizeOpenAIPassthroughOAuthBody(body, false)
+	require.NoError(t, err)
+	require.True(t, changed)
+
+	input := gjson.GetBytes(normalized, "input")
+	require.True(t, input.IsArray(), "object input should be wrapped in array")
+	items := input.Array()
+	require.Len(t, items, 1)
+	require.Equal(t, "message", items[0].Get("type").String())
+}
+
+func TestNormalizeOpenAIPassthroughOAuthBody_ArrayInputUnchanged(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","input":[{"type":"message","role":"user","content":"hi"}]}`)
+
+	normalized, _, err := normalizeOpenAIPassthroughOAuthBody(body, false)
+	require.NoError(t, err)
+
+	input := gjson.GetBytes(normalized, "input")
+	require.True(t, input.IsArray())
+	require.Len(t, input.Array(), 1)
+	require.Equal(t, "message", input.Array()[0].Get("type").String())
+}