feat(sandbox): add Rscript support to environment configuration

[zieen]

Add documentation and test coverage for Rscript language support in sandbox child process environment variables. Rscript benefits from existing lowercase proxy variables and libcurl/OpenSSL TLS certificate bundles.

Changes:

• Document Rscript compatibility with http_proxy/https_proxy
• Document SSL_CERT_FILE coverage for R and other OpenSSL tools
• Document CURL_CA_BUNDLE for R httr package
• Add test assertions verifying lowercase proxy vars
• Add test assertion verifying CURL_CA_BUNDLE

Summary

Related Issue

Changes

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[Copilot]

Pull request overview

This PR improves sandbox child-process environment configuration clarity and coverage for Rscript by documenting and testing the relevant proxy and TLS certificate bundle environment variables.

Changes:

• Document Rscript compatibility with lowercase http_proxy/https_proxy and libcurl/OpenSSL bundle env vars.
• Extend unit tests to assert lowercase proxy variables are present.
• Extend TLS env var test to assert CURL_CA_BUNDLE is set.

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[Copilot]

The new Rscript note is placed immediately above NODE_USE_ENV_PROXY, which could imply it relates to that setting. Since it documents the lowercase http_proxy/https_proxy entries, consider moving this comment next to those env var entries (or splitting into separate comments for Node vs. lowercase proxies) to keep the rationale aligned with the relevant keys.