Fix type annotation for collection in parse_owned function

[benracine]

Explicitly annotate the type of items collected from format_items to resolve
compiler error E0282. The type Box<_> ensures correct inference of the boxed
collection type. This resolves issues encountered during compilation with
Ferrocene.

Closes: #689

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.7%. Comparing base (4a74924) to head (42be41a). Report is 41 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #690 +/- ## ======================================= - Coverage 97.8% 97.7% -0.1% ======================================= Files 81 83 +2 Lines 9378 9001 -377 ======================================= - Hits 9169 8794 -375 + Misses 209 207 -2 ```

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