[codex] Handle PowerShell UTF-8 setup failures

[iceweasel-oai]

Fixes #12496.

Why

Windows sandboxed PowerShell commands can run under ConstrainedLanguage on some machines, especially enterprise-managed Windows environments. In that mode, our PowerShell command prelude could fail before every command because it directly assigned [Console]::OutputEncoding to UTF-8. The actual user command still ran, but Codex surfaced noisy Cannot set property. Property setting is supported only on core types in this language mode. output for every shell call.

What Changed

• Makes the PowerShell UTF-8 output encoding prelude best-effort by wrapping the assignment in try { ... } catch {}.
• Keeps the existing UTF-8 behavior when PowerShell allows the assignment.
• Adds focused tests for adding the prelude and avoiding duplicate prelude insertion.

Validation

• cargo fmt -p codex-shell-command
• cargo check -p codex-shell-command
• git diff --check
• Verified a local ConstrainedLanguage PowerShell probe prints only the command output with no property-setting error.
• Verified codex exec from a temporary chcp 437 context reports utf-8 / 65001 and preserves non-ASCII output (café, 漢字).