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.

.github/CODEOWNERS

@@ -2,6 +2,7 @@
 /codex-rs/core/ @openai/codex-core-agent-team
 /codex-rs/ext/extension-api/ @openai/codex-core-agent-team
 /codex-rs/prompts/ @openai/codex-core-agent-team
+/codex-rs/utils/path-uri/ @openai/codex-core-agent-team
 
 # Keep macOS AKV signing changes reviewed by Codex maintainers.
 /.github/actions/setup-akv-pkcs11-codesigning/ @openai/codex-core-agent-team