core: render remote environment cwd natively

[anp-oai]

Why

When app-server runs on Linux while the selected execution environment runs Windows, the model-visible <environment_context> renders the selected cwd using Linux host syntax, such as /C:/windows. This gives the model a path spelling that does not match the environment where commands actually execute.

Stacked on #28146.

What

• Keep selected environment cwd values as PathUri while building environment context.
• Render cwd text using the path convention represented by the URI, with the canonical URI as a fallback.
• Preserve compatibility with legacy TurnContextItem.cwd values when reconstructing and diffing context.
• Extend the Wine-backed remote Windows test to assert that the model sees powershell and C:\windows.
• Record the remaining host workspace-root projection with a TODO(anp) follow-up.

Testing

• just test -p codex-core context::environment_context::tests
• bazel test //codex-rs/core/tests/remote_env_windows:smoke-test --test_output=errors