Fix Windows exec policy test flake (#18304)

## Summary

This fixes a Windows-only failure in the exec policy multi-segment shell
test. The test was meant to verify that a compound shell command only
bypasses sandboxing when every parsed segment has an explicit exec
policy allow rule.

On Windows, the read-only sandbox setup is intentionally treated as
lacking sandbox protection, so the old fixture could take the approval
path before reaching the intended bypass assertion. The test now uses
the workspace-write sandbox policy, keeping the focus on the per-segment
bypass rule while preserving the expected bypass_sandbox false result
when only cat is explicitly allowed.

codex-rs/core/src/exec_policy_tests.rs

@@ -1343,7 +1343,7 @@ prefix_rule(pattern=["cat"], decision="allow")
                 policy_src: Some(policy_src.to_string()),
                 command: command.clone(),
                 approval_policy,
-                sandbox_policy: SandboxPolicy::new_read_only_policy(),
+                sandbox_policy: SandboxPolicy::new_workspace_write_policy(),
                 file_system_sandbox_policy: read_only_file_system_sandbox_policy(),
                 sandbox_permissions: SandboxPermissions::UseDefault,
                 prefix_rule: None,