Render turn diffs for foreign environment paths (#34654)

## Why

Turn diffs need to render paths from remote environments whose path convention may differ from the Codex host.

## What changed

- Preserve `PathUri` values through applied patch deltas, display-root discovery, and turn-diff tracking.
- Add host-independent relative-path rendering and normalize Windows drive-letter case in `PathUri`.
- Render paths relative to each environment's repository root while keeping Git diff headers slash-separated.

## Testing

- Cover POSIX, Windows, UNC, encoded-separator, and opaque URI relative paths.
- Verify an applied patch in a remote Windows working directory emits a portable unified diff.

GitOrigin-RevId: 526f70ae36a40627de5496d375310d12295b69bb

codex-rs/utils/path-uri/src/api_path_string_tests.rs

@@ -135,7 +135,7 @@ const RENDER_CASES: &[RenderCase] = &[
     RenderCase::round_trips(
         "file:///d:/snowman/%E2%98%83",
         PathConvention::Windows,
-        r"d:\snowman\☃",
+        r"D:\snowman\☃",
     ),
     RenderCase::round_trips("file:///C:/tmp/", PathConvention::Windows, "C:\\tmp\\"),
     RenderCase::round_trips(