Share Git safe-command logic on Windows

[iceweasel-oai]

Why

BUGB-15601 showed that the Windows safe-command path had drifted from the generic Git classifier. The Windows-specific Git parser could classify a PowerShell-wrapped git command as safe as soon as it found a safelisted subcommand, without applying the generic checks for unsafe subcommand options such as --output, --ext-diff, --textconv, --paginate, or cat-file --filters.

The generic classifier already models the Git command boundary and the read-only argument checks more carefully, so Windows should reuse that logic instead of maintaining a smaller parallel parser.

What Changed

• Extracted the existing generic Git classification logic into is_safe_git_command.
• Updated windows_safe_commands.rs to call that shared helper for parsed PowerShell git commands.
• Removed the Windows-only Git subcommand safelist, including the cat-file allowance that was part of the reported bypass.
• Added a Windows regression test that keeps PowerShell-wrapped Git commands with side-effecting options classified unsafe.
• Made the full-path PowerShell test discover the installed PowerShell executable instead of depending on one hard-coded pwsh.exe path.

Verification

• cargo test -p codex-shell-command rejects_git_subcommand_options_with_side_effects
• cargo test -p codex-shell-command git_global_override_flags_are_not_safe
• cargo test -p codex-shell-command windows_powershell_full_path_is_safe -- --nocapture

[evawong-oai]

This PR makes the shared Git classifier the Windows source of truth, but that classifier need to also consider pager options when they appear before the subcommand.

Examples:
git --paginate log -1
git -p log -1

The current check rejects --paginate only after the matched subcommand. Git treats this as a global option too, so find_git_subcommand skips it, then the read only arg check never sees it.

Since the PR is specifically consolidating Git safety logic, I think we should either reject -p and --paginate as unsafe global Git options in this PR, or create a clear follow up before landing.