[codex] Make AbsolutePathBuf joins infallible (#16981)

Having to check for errors every time join is called is painful and
unnecessary.

codex-rs/Cargo.lock

@@ -2878,7 +2878,6 @@ name = "codex-utils-absolute-path"
 version = "0.0.0"
 dependencies = [
  "dirs",
- "path-absolutize",
  "pretty_assertions",
  "schemars 0.8.22",
  "serde",