correct the index type in Boost interface

[dimpase]

without this fix, conda build of Sage 10.6 is failing;
this is with clang 18.1.8

This was reported on https://groups.google.com/g/sage-devel/c/YSiaVdsLKQg/m/13mZHNErCAAJ,
but the cause wasn't identified correctly.

📝 Checklist

• The title is concise and informative.
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• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
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⌛ Dependencies

[dimpase]

@saraedum @tobiasdiez

[github-actions]

Documentation preview for this PR (built with commit bfc87ec; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[dimpase]

@dcoudert - perhaps a better fix here would be to change the type on the cython side, not on the C++ side?

[tobiasdiez]

Is this the same problem that #39526 fixes?

[dimpase]

closing in favour of a better solution on #39526