path-uri: normalize parent segments in absolute joins (#29903)

## Why

`PathUri::join` normalized `..` for relative paths, but its
absolute-path branch rebuilt URIs through `url::PathSegmentsMut::push`,
which skips dot segments. `/tmp/a/../b` therefore resolved to `/tmp/a/b`
instead of `/tmp/b`.

## What changed

Normalize absolute native path segments before constructing the file
URI. Parent traversal now clamps at POSIX roots, Windows drive roots,
and UNC share roots, including paths with repeated separators.

Add platform-independent coverage for POSIX, drive, UNC, root-clamping,
and repeated-separator cases.

## Manual validation

- `just test -p codex-utils-path-uri`

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

@@ -18,8 +18,11 @@ use thiserror::Error;
 use ts_rs::TS;
 use url::Url;
 
+mod absolute_path_normalization;
 mod api_path_string;
 
+use absolute_path_normalization::path_uri_from_segments;
+
 pub use api_path_string::LegacyAppPathString;
 pub use api_path_string::LegacyAppPathStringError;
 
@@ -589,7 +592,7 @@ fn parse_posix_path(path: &str) -> Option<PathUri> {
             format!("/{path}").as_bytes(),
         ));
     }
-    path_uri_from_segments(/*host*/ None, path.split('/'))
+    path_uri_from_segments(PathConvention::Posix, /*host*/ None, path.split('/'))
 }
 
 fn parse_windows_path(path: &str) -> Option<PathUri> {
@@ -612,6 +615,7 @@ fn parse_windows_path(path: &str) -> Option<PathUri> {
             if drive.is_ascii_alphabetic() && is_windows_separator_byte(*separator)
     ) {
         return path_uri_from_segments(
+            PathConvention::Windows,
             /*host*/ None,
             std::iter::once(&path[..2]).chain(path[3..].split(is_windows_separator_char)),
         );
@@ -623,31 +627,17 @@ fn parse_windows_path(path: &str) -> Option<PathUri> {
         let mut components = path[2..].split(is_windows_separator_char);
         let host = components.next().filter(|host| !host.is_empty())?;
         let share = components.next().filter(|share| !share.is_empty())?;
-        return path_uri_from_segments(Some(host), std::iter::once(share).chain(components))
-            .or_else(|| Some(windows_opaque_path_uri(path)));
+        return path_uri_from_segments(
+            PathConvention::Windows,
+            Some(host),
+            std::iter::once(share).chain(components),
+        )
+        .or_else(|| Some(windows_opaque_path_uri(path)));
     }
 
     None
 }
 
-fn path_uri_from_segments<'a>(
-    host: Option<&str>,
-    segments: impl Iterator<Item = &'a str>,
-) -> Option<PathUri> {
-    let mut url = Url::parse("file:///").ok()?;
-    if let Some(host) = host {
-        url.set_host(Some(host)).ok()?;
-    }
-    {
-        let mut url_segments = url.path_segments_mut().ok()?;
-        url_segments.clear();
-        for segment in segments {
-            url_segments.push(segment);
-        }
-    }
-    PathUri::try_from(url).ok()
-}
-
 fn windows_opaque_path_uri(path: &str) -> PathUri {
     let path_bytes = path
         .encode_utf16()