Preserve Windows startup variables and MXC child environment defaults (#54027)

## Why

Windows MXC needs valid host startup variables, while an empty child environment must remain empty for the SDK to select Windows user-profile defaults.

## What changed

- Replace caller overrides of `SystemDrive` and `LOCALAPPDATA` case-insensitively with nonempty Unicode host values, failing before launch if either is unavailable.
- Track whether the original child environment was empty, ignoring launcher transport keys. Supply host `SystemRoot` for launcher bootstrap in that case, then clear the bootstrap environment before the SDK selects defaults.

## Testing

Extend transport tests to cover host-value replacement and preservation of empty child environments. Strengthen the PowerShell fallback RPC test to require `SystemDrive`, `LOCALAPPDATA`, `USERPROFILE`, and `PATH` in the child.

GitOrigin-RevId: a097e1079069ef897ebb5ba5e63546328c5faeaa

codex-rs/exec-server/tests/exec_process/windows_sandbox.rs

@@ -48,7 +48,7 @@ async fn powershell_alias_falls_back_without_primary_runtime_over_rpc(
                     .into_owned(),
                 "-NoProfile".to_owned(),
                 "-Command".to_owned(),
-                "Write-Output 'fallback-ok'".to_owned(),
+                "if (!$env:SystemDrive -or !$env:LOCALAPPDATA -or !$env:USERPROFILE -or !$env:PATH) { throw 'missing Windows defaults' }; Write-Output 'fallback-ok'".to_owned(),
             ],
             cwd,
             env_policy: None,