path-uri: render native paths across platforms (#27819)

## Why

We're moving to `PathUri` in more places to support cross-OS
app-server/exec-server, but we don't want to expose the URI encoding to
users of app-server's public APIs yet.

We'll need to translate at the app-server API boundary between
client-visible "regular" paths that are appropriate for the OS of the
environment for which the paths make sense, which means using the
environment's path personality to do the conversion.

`PathUri` doesn't yet attempt to encode environment ID, so for now we'll
sniff the most likely path convention for a given path.

## What

- Add `PathConvention` and `NativePathString` with host-independent
POSIX, Windows drive, and UNC rendering.
- Cover cross-host rendering, encoding, Unicode, invalid components.

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

@@ -58,6 +58,42 @@ fn file_uri_parses_a_windows_path_on_any_host() {
     );
 }
 
+#[test]
+fn infers_path_conventions_from_uri_shape() {
+    for (uri, expected) in [
+        ("file:///", Some(PathConvention::Posix)),
+        ("file:///home/alice/src", Some(PathConvention::Posix)),
+        ("file:///C:/Users/Alice/src", Some(PathConvention::Windows)),
+        ("file:///d:", Some(PathConvention::Windows)),
+        ("file://server/share/src", Some(PathConvention::Windows)),
+        // Opaque fallback for POSIX bytes `/tmp/null-\0-\xff-byte`.
+        (
+            "file:///%00/bad/path/L3RtcC9udWxsLQAt_y1ieXRl",
+            Some(PathConvention::Posix),
+        ),
+        // Opaque fallback for Windows UTF-16LE `\\.\COM1\`.
+        (
+            "file:///%00/bad/path/XABcAC4AXABDAE8ATQAxAFwA",
+            Some(PathConvention::Windows),
+        ),
+        ("file:///%00/bad/path/YQ", None),
+    ] {
+        let path = PathUri::parse(uri).expect("valid path URI");
+
+        assert_eq!(path.infer_path_convention(), expected, "inferring {uri}");
+    }
+}
+
+#[test]
+fn drive_shaped_posix_uri_is_intentionally_inferred_as_windows() {
+    let path = PathUri::parse("file:///C:/actually/a/posix/path").expect("valid path URI");
+
+    // `/C:/...` is valid on POSIX, but treating this uncommon spelling as a
+    // Windows drive lets callers render the overwhelmingly more common foreign
+    // Windows URI without separately carrying its source convention.
+    assert_eq!(path.infer_path_convention(), Some(PathConvention::Windows));
+}
+
 #[cfg(windows)]
 #[test]
 fn file_uri_falls_back_for_windows_prefixes_without_a_uri_representation() {