[ffi] Set default parameter name in using callback

[dickermoshe]

This is an extremely minor change. Providing a argument name makes auto complete better.

Before:

Recording.2025-04-28.133050.mp4

After:

Recording.2025-04-28.132959.mp4

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[dcharkes]

Thanks @dickermoshe!

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
ffi	None	2.1.4	2.1.4	2.1.4	✔️

Changelog Entry ❗

Package	Changed Files
package:ffi	pkgs/ffi/lib/src/arena.dart

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API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols
ffi	MallocAllocator CallocAllocator

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[dcharkes]

If you want a release of the package, please also bump the version and add an entry to the changelog.