tests: check baked room server package (#269)

Fixes the merged symphony-codex eval assertion so it checks the actual
package object in the rendered image package list. The baked package is
exposed as `pkgs.symphony-room-server`, while its derivation name is
`room-server-wrapped`.

Validation:
- `nix run nixpkgs#nixfmt-rfc-style -- --check tests/default.nix`
- `nix --accept-flake-config --option eval-cache false eval --raw
.#checks.x86_64-linux.eval.drvPath`

Author: harivansh-afk

tests/default.nix

@@ -2324,7 +2324,7 @@ let
         message = "symphony-codex image should publish an immutable production tag";
       }
       {
-        assertion = builtins.elem "symphony-room-server" symphonyCodex.packageNames;
+        assertion = builtins.elem pkgs.symphony-room-server symphonyCodex.packages;
         message = "symphony-codex image should include the room-server binary it starts";
       }
       {