Restore rb_sys dependency for Rust

[bangseongbeom]

What was the end-user or developer problem that led to this PR?

After running bundle gem --ext=rust my_gem and bundle install, I got an error when I run bundle exec rake compile (I followed oxidize.rb guide):

$ bundle exec rake compile
rake aborted!
LoadError: cannot load such file -- rb_sys/extensiontask (LoadError)
/.../Rakefile:12:in '<top (required)>'
/.../rake:27:in '<top (required)>'
(See full trace by running task with --trace)

What is your fix for the problem, implemented in this PR?

Revert #8455 partially. In that PR, rb_sys was removed from the dependencies, but it seems to be unintended. Without rb_sys, rb_sys/extensiontask cannot be loaded, so all rake commands cannot be run.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[Copilot]

Pull request overview

Restores the rb_sys gem dependency in Bundler’s Rust gem scaffolding so the generated Rakefile (which requires rb_sys/extensiontask) works after bundle install, fixing LoadError failures when running tasks like bundle exec rake compile.

Changes:

• Add rb_sys as a dependency in the Rust newgem gemspec template.
• Update bundle gem --ext=rust specs to assert the generated gemspec includes rb_sys.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
bundler/spec/commands/newgem_spec.rb	Updates Rust scaffold expectations to include an rb_sys dependency in the generated gemspec.
bundler/lib/bundler/templates/newgem/newgem.gemspec.tt	Adds rb_sys dependency when generating a gem with --ext=rust.

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