fix(compact): 识别 /v1/responses body 中的 compaction_trigger 信号

Codex remote compact v2 可以把 compact 触发器作为 input item
（type=compaction_trigger）嵌入普通 POST /v1/responses 请求体，
而非直接调用 /v1/responses/compact。此前 isCompactRequest 仅检查
URL path 后缀，导致 body-signal 形式的 compact 请求被当作普通
Responses 处理——上游路径、模型映射、body 归一化全部错误，Codex
收到非 compact 响应后报 "expected exactly one compaction output
item, got 0"，长会话无法继续。

新增 hasCompactionTriggerInInput 检测 input 中的 compaction_trigger
item，命中后提升为 compact 请求并改写 URL path，使后续所有
isCompactRequest 分支（模型映射、body 归一化、上游 URL 构建）
自动生效。

Fixes #3777 (part 1: body-signal routing)

backend/internal/service/openai_compact_body_signal.go

@@ -0,0 +1,30 @@
+package service
+
+import "github.com/tidwall/gjson"
+
+// hasCompactionTriggerInInput detects the Codex remote compact v2 body signal:
+// an input item with type "compaction_trigger". When the client sends this
+// inside a normal POST /v1/responses (instead of POST /v1/responses/compact),
+// the request must still be treated as a compact request — otherwise the
+// upstream path, model mapping, and body normalization are all wrong, causing
+// Codex to receive a non-compact response and fail with:
+//
+//	"remote compaction v2 expected exactly one compaction output item, got 0"
+func hasCompactionTriggerInInput(body []byte) bool {
+	if len(body) == 0 {
+		return false
+	}
+	input := gjson.GetBytes(body, "input")
+	if !input.IsArray() {
+		return false
+	}
+	found := false
+	input.ForEach(func(_, item gjson.Result) bool {
+		if item.Get("type").String() == "compaction_trigger" {
+			found = true
+			return false
+		}
+		return true
+	})
+	return found
+}

backend/internal/service/openai_compact_body_signal_test.go

@@ -0,0 +1,56 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestHasCompactionTriggerInInput_DetectsCompactSignal(t *testing.T) {
+	body := []byte(`{
+		"model":"gpt-5.5",
+		"stream":true,
+		"input":[
+			{"type":"message","role":"user","content":"hello"},
+			{"type":"compaction_trigger"}
+		]
+	}`)
+	require.True(t, hasCompactionTriggerInInput(body))
+}
+
+func TestHasCompactionTriggerInInput_NoTrigger(t *testing.T) {
+	body := []byte(`{
+		"model":"gpt-5.5",
+		"input":[
+			{"type":"message","role":"user","content":"hello"}
+		]
+	}`)
+	require.False(t, hasCompactionTriggerInInput(body))
+}
+
+func TestHasCompactionTriggerInInput_EmptyInput(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.5","input":[]}`)
+	require.False(t, hasCompactionTriggerInInput(body))
+}
+
+func TestHasCompactionTriggerInInput_NoInputField(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.5"}`)
+	require.False(t, hasCompactionTriggerInInput(body))
+}
+
+func TestHasCompactionTriggerInInput_EmptyBody(t *testing.T) {
+	require.False(t, hasCompactionTriggerInInput(nil))
+	require.False(t, hasCompactionTriggerInInput([]byte{}))
+}
+
+func TestHasCompactionTriggerInInput_StringInput(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.5","input":"compaction_trigger"}`)
+	require.False(t, hasCompactionTriggerInInput(body))
+}
+
+func TestHasCompactionTriggerInInput_CompactTriggerOnly(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.5","input":[{"type":"compaction_trigger"}]}`)
+	require.True(t, hasCompactionTriggerInInput(body))
+}

backend/internal/service/openai_gateway_service.go

@@ -2777,6 +2777,19 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 	}
 	upstreamModel := billingModel
 	isCompactRequest := isOpenAIResponsesCompactPath(c)
+	// Body-signal detection: Codex remote compact v2 can send the compact
+	// trigger inside a normal POST /v1/responses body (input item with
+	// type "compaction_trigger") instead of calling /v1/responses/compact
+	// directly. Detect and promote to compact request (#3777).
+	if !isCompactRequest && hasCompactionTriggerInInput(body) {
+		isCompactRequest = true
+		// Rewrite the request path so downstream URL builders
+		// (appendOpenAIResponsesRequestPathSuffix) route to /responses/compact.
+		if c != nil && c.Request != nil && c.Request.URL != nil {
+			c.Request.URL.Path = strings.TrimRight(c.Request.URL.Path, "/") + "/compact"
+		}
+		logger.LegacyPrintf("service.openai_gateway", "[OpenAI] Compact body-signal detected: input contains compaction_trigger (account: %s)", account.Name)
+	}
 	compactMapped := false
 	if isCompactRequest {
 		compactMappedModel := resolveOpenAICompactForwardModel(account, billingModel)