refactor(line): hoist body size const, add boundary tests

- Move maxWebhookBodySize to package-level const
- Add TestWebhookAcceptsMaxBodySize (exact limit → 403, not 413)
- Add TestWebhookRejectsOversizedBodyBeforeSignatureCheck
- Use const in test instead of magic number

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/channels/line/line.go

@@ -32,6 +32,10 @@ const (
 	lineBotInfoEndpoint  = lineAPIBase + "/info"
 	lineLoadingEndpoint  = lineAPIBase + "/chat/loading/start"
 	lineReplyTokenMaxAge = 25 * time.Second
+
+	// Limit request body to prevent memory exhaustion (DoS).
+	// LINE webhook payloads are typically a few KB; 1 MiB is generous.
+	maxWebhookBodySize = 1 << 20 // 1 MiB
 )
 
 type replyTokenEntry struct {
@@ -166,9 +170,6 @@ func (c *LINEChannel) webhookHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Limit request body to prevent memory exhaustion (DoS).
-	// LINE webhook payloads are typically a few KB; 1 MB is generous.
-	const maxWebhookBodySize = 1 << 20 // 1 MB
 	body, err := io.ReadAll(io.LimitReader(r.Body, maxWebhookBodySize+1))
 	if err != nil {
 		logger.ErrorCF("line", "Failed to read request body", map[string]any{

pkg/channels/line/line_test.go

@@ -11,8 +11,7 @@ import (
 func TestWebhookRejectsOversizedBody(t *testing.T) {
 	ch := &LINEChannel{}
 
-	// Create a body larger than maxWebhookBodySize (1 MB)
-	oversized := bytes.Repeat([]byte("A"), (1<<20)+1)
+	oversized := bytes.Repeat([]byte("A"), maxWebhookBodySize+1)
 	req := httptest.NewRequest(http.MethodPost, "/webhook", bytes.NewReader(oversized))
 	rec := httptest.NewRecorder()
 
@@ -23,6 +22,36 @@ func TestWebhookRejectsOversizedBody(t *testing.T) {
 	}
 }
 
+func TestWebhookAcceptsMaxBodySize(t *testing.T) {
+	ch := &LINEChannel{}
+
+	body := bytes.Repeat([]byte("A"), maxWebhookBodySize)
+	req := httptest.NewRequest(http.MethodPost, "/webhook", bytes.NewReader(body))
+	rec := httptest.NewRecorder()
+
+	ch.webhookHandler(rec, req)
+
+	// Missing signature should be rejected, but the body size should not trigger 413.
+	if rec.Code != http.StatusForbidden {
+		t.Errorf("expected status %d, got %d", http.StatusForbidden, rec.Code)
+	}
+}
+
+func TestWebhookRejectsOversizedBodyBeforeSignatureCheck(t *testing.T) {
+	ch := &LINEChannel{}
+
+	oversized := bytes.Repeat([]byte("A"), maxWebhookBodySize+1)
+	req := httptest.NewRequest(http.MethodPost, "/webhook", bytes.NewReader(oversized))
+	req.Header.Set("X-Line-Signature", "invalidsignature")
+	rec := httptest.NewRecorder()
+
+	ch.webhookHandler(rec, req)
+
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Errorf("expected status %d, got %d", http.StatusRequestEntityTooLarge, rec.Code)
+	}
+}
+
 func TestWebhookRejectsNonPostMethod(t *testing.T) {
 	ch := &LINEChannel{}