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/utils/path-uri/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "codex-utils-path-uri"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-utils-absolute-path = { workspace = true }
+schemars = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+thiserror = { workspace = true }
+ts-rs = { workspace = true, features = ["no-serde-warnings"] }
+url = { workspace = true }
+urlencoding = { workspace = true }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+serde_json = { workspace = true }
+
+[lib]
+doctest = false