fix: log a warning on duplicate attachment filenames in lookup() (dpycache-2)

lookup()'s {filename: FileRef} comprehension silently kept only the last
attachment when a foreign message carried duplicate filenames (Discord
permits this; cache()'s own upload path already rejects it loudly, so the
gap is only a foreign message passed to lookup()). Refs still resolve
correctly by attachment_id, so keep the last-wins map (non-breaking) but log
the collision instead of staying silent about it.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/dpy_cache/dpy_cache.py

@@ -200,12 +200,23 @@ class DPYCache:
     async def lookup(self, message: "str | int | discord.Message") -> dict[str, FileRef]:
         """reverse lookup: a jump-url, message id (``int`` or a digit-only ``str``, matching
         the snowflake shape stored in JSON/DBs), or Message -> {filename: FileRef}; raises
-        (NotFound) if the message is gone, never an empty map for a missing message"""
+        (NotFound) if the message is gone, never an empty map for a missing message
+
+        keyed by filename, so a foreign message with duplicate attachment filenames (Discord
+        allows this; ``cache()`` itself never produces one) collapses to the last attachment
+        of that name in the map - logged, not silent. every ref still resolves correctly by
+        its own ``attachment_id`` regardless"""
         resolved = await self._resolve_message(message)
-        return {
-            attachment.filename: _ref_from_attachment(attachment.filename, attachment, resolved)
-            for attachment in resolved.attachments
-        }
+        refs: dict[str, FileRef] = {}
+        for attachment in resolved.attachments:
+            if attachment.filename in refs:
+                log.warning(
+                    "dpy_cache: message %s has more than one attachment named %r; "
+                    "lookup() keeps only the last (attachment_id=%s)",
+                    resolved.id, attachment.filename, attachment.id,
+                )
+            refs[attachment.filename] = _ref_from_attachment(attachment.filename, attachment, resolved)
+        return refs
 
     def _normalize(self, files: "dict[str, FileContent] | list[FileContent]") -> "list[tuple[str, FileContent]]":
         """normalize dict/list input to an ordered list of (name, source); raises ValueError