Bridge command generation to powershell when on Windows (#2319)

## What? Why? How?
- When running on Windows, codex often tries to invoke bash commands,
which commonly fail (unless WSL is installed)
- Fix: Detect if powershell is available and, if so, route commands to
it
- Also add a shell_name property to environmental context for codex to
default to powershell commands when running in that environment

## Testing
- Tested within WSL and powershell (e.g. get top 5 largest files within
a folder and validated that commands generated were powershell commands)
- Tested within Zsh
- Updated unit tests

---------

Co-authored-by: Eddy Escardo <eddy@openai.com>

codex-rs/core/Cargo.toml

@@ -71,6 +71,9 @@ openssl-sys = { version = "*", features = ["vendored"] }
 [target.aarch64-unknown-linux-musl.dependencies]
 openssl-sys = { version = "*", features = ["vendored"] }
 
+[target.'cfg(target_os = "windows")'.dependencies]
+which = "6"
+
 [dev-dependencies]
 assert_cmd = "2"
 core_test_support = { path = "tests/common" }