fix(internal/librarian/rust): map recursive fields to Box

[olavloite]

Recursive fields that are not repeated or oneof should be mapped to Box.

[coryan]

Thanks!

[gemini-code-assist]

Code Review

This pull request introduces logic to detect when prost will box a recursive field and updates the code generation templates accordingly. The core detection logic in mapToBoxed appears sound. However, there are a couple of issues. A test case for a map with a recursive value has an incorrect expectation for whether the field will be boxed. Additionally, the new template logic for converting to the prost type is missing a necessary dereference for boxed types, which will cause a compilation error. I've left specific comments with suggestions to address these points.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.93%. Comparing base (0a5722d) to head (8ca9c5f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4074   +/-   ##
=======================================
  Coverage   81.93%   81.93%           
=======================================
  Files          78       78           
  Lines        6516     6516           
=======================================
  Hits         5339     5339           
  Misses        832      832           
  Partials      345      345           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.