apply-patch: carry paths as PathUri (#28854)

## Why

Allows the model to edit files that are hosted on a different OS than
where app-server is running.

## What

* Use `PathUri` for apply_patch-internal data structures
* Limit `PathUri` -> `AbsolutePathBuf` conversion to cases where the
inferred path convention matches the host OS, allows requiring valid
paths to pass to perms check
* Adds `PathConvention::path_segments()` for iterating over path
segments regardless of OS
* Handle cross-platform relative paths in path filename parsing for
sniffing a shell
* Ensure we can apply patches in the wine e2e test

codex-rs/apply-patch/src/standalone_executable.rs

@@ -65,6 +65,8 @@ pub fn run_main() -> i32 {
             return 1;
         }
     };
+    // TODO(anp): Discover the standalone executable cwd as PathUri directly.
+    let cwd = codex_utils_path_uri::PathUri::from_abs_path(&cwd);
     match runtime.block_on(crate::apply_patch(
         &patch_arg,
         &cwd,