exec-server: preserve empty workspace roots

[pakrym-oai]

Depends on #31892.

Why

Exec-server sandbox contexts carry the concrete roots that define symbolic :workspace_roots authority. Both process and filesystem sandboxing treated an empty root list as if it were missing and rebound it to the sandbox cwd. A caller that intentionally selected no workspace roots could therefore still access cwd files.

What changed

• Stop substituting sandbox cwd when process or filesystem workspace roots are empty.
• Keep the existing from_permission_profile_with_cwd convenience behavior by carrying cwd as an explicit workspace root.
• Add remote exec-server integration coverage proving both fs/readFile and a sandboxed process cannot read a cwd file after workspace roots are explicitly cleared.

Testing

• just test -p codex-exec-server --test file_system_unix remote_read_file_preserves_empty_workspace_roots
• just test -p codex-exec-server --test exec_process remote_process_preserves_empty_workspace_roots

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 013e18645e

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".