fix: Fix tilde expansion to avoid absolute-path escape

[tiffanycitra]

Motivation

• Prevent inputs like ~// or ~///etc from expanding to arbitrary absolute paths (e.g. /) because Path::join discards the left side when the right side is absolute, which could allow config values to escape HOME and broaden writable roots.

Description

• In codex-rs/utils/absolute-path/src/lib.rs update maybe_expand_home_directory to trim leading separators from the suffix and return home when the remainder is empty so tilde expansion stays rooted under HOME.
• Add a non-Windows unit test home_directory_double_slash_on_non_windows_is_expanded_in_deserialization that validates "~//code" expands to home.join("code").

Testing

• Ran just fmt successfully.
• Ran just fix -p codex-utils-absolute-path (Clippy autofix) successfully.
• Ran cargo test -p codex-utils-absolute-path and all tests passed.

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Codex Task

[etraut-openai]

Good catch!