Never match a date separator as a summary anchor

Date separators are local MessageObjects that live in the same
ChatActivity.messages list as real messages, and ChatActivity builds them
with messageOwner.id left at 0 (their stable ids come from a separate
dateObjectsStableIds space, so they never collide with injected cards).

find_index_for_anchor scanned for an exact id match without requiring a
positive id, so looking up anchor 0 returned the index of the first
separator and would have placed a card against a divider rather than a
message. loaded_message_ids had the matching flaw: it recorded the
separators' 0, which made a summary with anchor id 0 look restorable.

Reject non-positive anchors outright and keep separators out of the loaded
id set. Every real chat interleaves separators, so the harness now models
them: they must not be picked as the insertion point, must not satisfy an
anchor-loaded check, and summaries anchored on real messages must still
restore correctly with separators in between.

src/services/rich_injector.py

@@ -195,6 +195,12 @@ def find_index_for_anchor(messages_list: Any, anchor_msg_id: int, require_anchor
     again once the anchor is paged in.
     """
     anchor_msg_id = int(anchor_msg_id)
+    # Anchors are real, positive message ids. A non-positive anchor must never
+    # match: date separators are local MessageObjects whose messageOwner.id is
+    # 0, so scanning for 0 would "find" the first separator and place the card
+    # against a divider instead of a message.
+    if anchor_msg_id <= 0:
+        return -1
     try:
         size = messages_list.size()
         for i in range(size):
@@ -558,8 +564,13 @@ def loaded_message_ids(messages_list: Any) -> set:
     try:
         for i in range(messages_list.size()):
             owner = getattr(messages_list.get(i), "messageOwner", None)
-            if owner is not None:
-                ids.add(int(getattr(owner, "id", 0) or 0))
+            if owner is None:
+                continue
+            msg_id = int(getattr(owner, "id", 0) or 0)
+            # Skip date separators (id 0). Keeping them would make a summary
+            # whose anchor id is 0 look restorable.
+            if msg_id:
+                ids.add(msg_id)
     except Exception as exc:
         diagnostics.record_error("injector.loaded_message_ids", exc)
     return ids