fix: build PATH env var using OsString instead of String

[bolinfest]

Why

prepend_path_entry_for_codex_aliases() was rebuilding PATH through std::env::var("PATH") and String formatting. That assumes the inherited PATH is valid UTF-8, which is not guaranteed on supported platforms. When PATH contains non-UTF-8 bytes, Codex can fail to prepend the per-session helper directory even though the existing environment is otherwise usable.

What changed

• read PATH with std::env::var_os() instead of std::env::var()
• construct the updated value as an OsString instead of a formatted String
• preserve the existing PATH contents while still prepending the arg0 helper directory

How to verify

• Not run locally; change is limited to how PATH is reconstructed before std::env::set_var().

---

Stack created with Sapling. Best reviewed with ReviewStack.

• fix: make remaining PATH reads OsString-aware #15363
• -> fix: build PATH env var using OsString instead of String #15360

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Do not leave a non-Unicode PATH in the process env

On Unix, var_os("PATH") can return raw bytes, so this now preserves a non-Unicode PATH in the current process. Startup still succeeds, but later code paths still assume the environment is UTF-8: core/src/exec_env.rs:24 (used by the shell tool in core/src/tools/handlers/shell.rs:75 and unified exec in core/src/unified_exec/process_manager.rs:586) and shell-escalation/src/unix/escalate_client.rs:48 both iterate with std::env::vars(), which panics when PATH is not Unicode. Before this change the Err(NotUnicode) branch replaced PATH with the helper dir's UTF-8 string, so those flows kept working; with this version, users who launch Codex from a shell with a non-UTF8 path entry can crash as soon as they run a shell or escalated command.

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[bolinfest]

I'll address this in a follow-up.