fix: add archived query parameter to chat list endpoint (#22562)

Despite the SDK type having an `Archived` field for chats, this data was
never fetched from the database — the `GetChatsByOwnerID` query
hardcoded `AND archived = false`, and the `convertChat` function never
mapped the field.

This PR adds an optional `archived` query parameter to `GET
/api/experimental/chats`:

| Value | Behavior |
|-------|----------|
| *(not provided)* | Returns all chats (active and archived) |
| `archived=false` | Returns only non-archived chats |
| `archived=true` | Returns only archived chats |

This follows the same pattern used by template versions
(`sqlc.narg('archived')` nullable boolean).

Also fixes `convertChat` to populate the `Archived` field in API
responses, which was never being set despite existing on the SDK type.

coderd/database/queries/chats.sql

@@ -110,7 +110,10 @@ FROM
     chats
 WHERE
     owner_id = @owner_id::uuid
-    AND archived = false
+    AND CASE
+        WHEN sqlc.narg('archived') :: boolean IS NULL THEN true
+        ELSE chats.archived = sqlc.narg('archived') :: boolean
+    END
 ORDER BY
     updated_at DESC;