fix: split single-string proxy invocations

[tsubasakong]

Summary

• normalize rtk proxy args when the shell passes the whole command as one string
• split that single argument with shell parsing before spawning the child process
• add regression tests for quoted single-string and already-split invocations

Fixes #388

Validation

• git diff --check
• cargo toolchain not available on this host, so Rust tests could not be run locally

[pszymkowiak]

Hi! Thanks for the contribution! Since March 6, all PRs should target the develop branch instead of master (see CONTRIBUTING.md).

Could you update the base branch? Click Edit at the top right of this PR and change it from master to develop.

Thanks!

[tsubasakong]

Closing this one because #480 already landed the #388 fix on develop (quoted rtk proxy arg splitting + tests), so retargeting this PR now would be duplicate work. Thanks for the review.