Fix fragment exports for older Xray clients (#6702)

* Fix fragment exports for older Xray clients

* fix(link): tolerate a finalmask without tcp in panel share links

withLegacyFragmentRanges called finalmask.tcp.map unguarded, but stored
rows reach the link generator unparsed and dropEmptyFinalMask deletes an
empty tcp list on save. Any VMess/VLESS/Trojan/SS inbound with only UDP
masks or quicParams threw a TypeError in the QR, info and export-links
views. The Go counterpart already skipped a missing tcp.

Also trims the Go helper's comment to the two-line cap and drops a
[]string branch no JSON-decoded finalmask can reach.

---------

Co-authored-by: Artem K <a.kush@vkteam.ru>
Co-authored-by: Sanaei <ho3ein.sanaei@gmail.com>

internal/sub/service_test.go

@@ -3,6 +3,7 @@ package sub
 import (
 	"encoding/base64"
 	"encoding/json"
+	"reflect"
 	"strings"
 	"testing"
 
@@ -1055,6 +1056,102 @@ func TestMarshalFinalMask_WithContent(t *testing.T) {
 	}
 }
 
+func TestMarshalFinalMaskAddsLegacyFragmentRanges(t *testing.T) {
+	lengths := []any{"5-10", "10-15", "15-20", "20-25", "25-30"}
+	delays := []any{"10-20", "5-20", "5-25", "15-25", "10-30"}
+	screenshotSettings := map[string]any{
+		"packets":  "tlshello",
+		"lengths":  lengths,
+		"delays":   delays,
+		"maxSplit": "10-15",
+	}
+	explicitSettings := map[string]any{
+		"length":  "1-2",
+		"lengths": []any{"8-9"},
+		"delay":   "3-4",
+		"delays":  []any{"6-7"},
+	}
+	emptySettings := map[string]any{
+		"lengths": []any{},
+		"delays":  []any{},
+	}
+	legacyOnlySettings := map[string]any{"length": "40-50", "delay": "10-20"}
+	otherSettings := map[string]any{"lengths": []any{"30-40"}}
+	fm := map[string]any{
+		"tcp": []any{
+			map[string]any{"type": "fragment", "settings": screenshotSettings},
+			map[string]any{"type": "fragment", "settings": explicitSettings},
+			map[string]any{"type": "fragment", "settings": emptySettings},
+			map[string]any{"type": "fragment", "settings": legacyOnlySettings},
+			map[string]any{"type": "sudoku", "settings": otherSettings},
+		},
+	}
+	original, err := json.Marshal(fm)
+	if err != nil {
+		t.Fatalf("marshal input finalmask: %v", err)
+	}
+
+	encoded, ok := marshalFinalMask(fm)
+	if !ok {
+		t.Fatal("expected finalmask with fragment masks to be marshaled")
+	}
+	var got map[string]any
+	if err := json.Unmarshal([]byte(encoded), &got); err != nil {
+		t.Fatalf("unmarshal marshaled finalmask: %v", err)
+	}
+	masks, _ := got["tcp"].([]any)
+	if len(masks) != 5 {
+		t.Fatalf("tcp mask count = %d, want 5", len(masks))
+	}
+	settingsAt := func(index int) map[string]any {
+		t.Helper()
+		mask, _ := masks[index].(map[string]any)
+		settings, _ := mask["settings"].(map[string]any)
+		if settings == nil {
+			t.Fatalf("tcp[%d] settings missing: %#v", index, mask)
+		}
+		return settings
+	}
+
+	gotScreenshot := settingsAt(0)
+	if gotScreenshot["length"] != "25-30" || gotScreenshot["delay"] != "10-30" {
+		t.Fatalf("legacy ranges = (%v, %v), want last array entries", gotScreenshot["length"], gotScreenshot["delay"])
+	}
+	if !reflect.DeepEqual(gotScreenshot["lengths"], lengths) || !reflect.DeepEqual(gotScreenshot["delays"], delays) {
+		t.Fatalf("per-segment ranges changed: lengths=%#v delays=%#v", gotScreenshot["lengths"], gotScreenshot["delays"])
+	}
+	if gotScreenshot["packets"] != "tlshello" || gotScreenshot["maxSplit"] != "10-15" {
+		t.Fatalf("other fragment settings changed: %#v", gotScreenshot)
+	}
+
+	gotExplicit := settingsAt(1)
+	if gotExplicit["length"] != "1-2" || gotExplicit["delay"] != "3-4" {
+		t.Fatalf("explicit legacy ranges were overwritten: %#v", gotExplicit)
+	}
+	gotEmpty := settingsAt(2)
+	if _, exists := gotEmpty["length"]; exists {
+		t.Fatalf("empty lengths must not emit a fallback: %#v", gotEmpty)
+	}
+	if _, exists := gotEmpty["delay"]; exists {
+		t.Fatalf("empty delays must not emit a fallback: %#v", gotEmpty)
+	}
+	gotLegacyOnly := settingsAt(3)
+	if gotLegacyOnly["length"] != "40-50" || gotLegacyOnly["delay"] != "10-20" {
+		t.Fatalf("legacy-only ranges changed: %#v", gotLegacyOnly)
+	}
+	if _, exists := settingsAt(4)["length"]; exists {
+		t.Fatalf("non-fragment mask received a fallback: %#v", settingsAt(4))
+	}
+
+	after, err := json.Marshal(fm)
+	if err != nil {
+		t.Fatalf("marshal input finalmask after export: %v", err)
+	}
+	if string(after) != string(original) {
+		t.Fatalf("marshalFinalMask mutated its input:\nbefore: %s\nafter:  %s", original, after)
+	}
+}
+
 func TestMarshalFinalMask_UnknownTypeIsDropped(t *testing.T) {
 	fm := map[string]any{
 		"tcp": []any{