fix(mypage): populate user_item_list from viewer.Items

MyPageTask.Parse (Wizard/MyPageTask.cs:155-163) does
`_userItemDict.Clear();` the moment `user_item_list` is present in
the response body — not when it's non-empty — then re-populates
from the wire. Our /mypage/index was emitting [] by default (the
field initializer on the DTO), which wiped the inventory that
/load/index had just populated.

Downstream consequence: the client's PackChildGachaInfo.CostGoodsCount
reads from _userItemDict, so a wiped dict makes every ticket-cost
pack report CostGoodsCount=0, PackConfig.EnableBuyPack returns false,
and is_hide=1 packs (including the tutorial legendary starter 99047)
disappear from the rotation pack list — even though the gift bundle
just granted the ticket and the DB row exists. The tutorial then
auto-selects whatever non-tutorial pack happens to be at index 0 of
the filtered list, the user can't afford it, and the flow is stuck.

Fix:

- MyPageController.Index now sets UserItemList from viewer.Items
  (already loaded by GetViewerByShortUdid's home-screen graph).
- DTO docstring rewritten to call out the presence-sensitive semantics
  and the load-bearing path through PackConfig.EnableBuyPack, so the
  next developer doesn't get the "empty is fine" hint the old comment
  implied.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Models/Dtos/Responses/MyPageIndexResponse.cs

@@ -267,8 +267,14 @@ public class MyPageIndexResponse
     // ── Per-viewer / event state ───────────────────────────────────────────
 
     /// <summary>
-    /// Updated item counts. Empty list = "no items to update" (client iterates 0 times, no UI change).
-    /// Per-viewer state — populate from viewer.Items when that wiring lands.
+    /// Full snapshot of the viewer's owned items — NOT a delta. The client's
+    /// <c>MyPageTask.Parse</c> (line 155-163) clears <c>_userItemDict</c> the moment it sees
+    /// this key, then re-populates from the wire list. Emitting <c>[]</c> wipes whatever
+    /// /load/index populated, breaking any client logic that reads from the dict — most
+    /// load-bearingly <c>PackChildGachaInfo.CostGoodsCount</c>, which gates tutorial-pack
+    /// visibility via <c>PackConfig.EnableBuyPack</c>. Controllers MUST populate the full
+    /// owned-items snapshot from <c>viewer.Items</c>; an empty list is correct only when the
+    /// viewer genuinely owns nothing.
     /// </summary>
     [JsonPropertyName("user_item_list")]
     [Key("user_item_list")]