Add typed file URIs (#26840)

## Why

Codex needs stable `file:` URI identifiers that can cross process and
operating-system boundaries without eagerly interpreting them as native
paths. Existing fields also need to keep accepting absolute path strings
during migration.

## What changed

- Add `codex-utils-path-uri` with a validated, immutable `PathUri`
wrapper that currently accepts only `file:` URLs.
- Expose URI-level `basename`, `parent`, and `join` operations that
preserve authorities and percent encoding without guessing the source
operating system.
- Keep native conversion explicit through `AbsolutePathBuf` and the
current host rules.
- Serialize as canonical URI text while accepting both URI text and
legacy absolute native paths during deserialization.
- Add adversarial coverage for Windows-looking and POSIX paths, UNC
authorities, encoded metadata characters, non-UTF-8 POSIX paths, URI
hierarchy operations, and legacy serde round trips.

codex-rs/Cargo.lock

@@ -4152,6 +4152,21 @@ dependencies = [
  "tempfile",
 ]
 
+[[package]]
+name = "codex-utils-path-uri"
+version = "0.0.0"
+dependencies = [
+ "codex-utils-absolute-path",
+ "pretty_assertions",
+ "schemars 0.8.22",
+ "serde",
+ "serde_json",
+ "thiserror 2.0.18",
+ "ts-rs",
+ "url",
+ "urlencoding",
+]
+
 [[package]]
 name = "codex-utils-plugins"
 version = "0.0.0"