test: cover rustup tool update execution

[iancleary]

Summary

• exercise forge tool update rustup through the CLI integration-test boundary
• verify the configured rustup subprocess receives the update argument
• verify the command reports a successful rustup result in JSON output

Context

The rustup update plan and documentation are already present on main. This adds the missing non-dry-run regression proof so the behavior requested in #97 stays covered end to end.

Closes #97

Testing

• cargo fmt --check
• cargo test -p forge --test cli cli_tool_update_rustup_runs_rustup_update
• cargo test -p forge
• cargo check