TUI: prefer CODEX_HOME for IDE IPC

[etraut-openai]

Why

The IDE extension is moving its Unix IPC socket from /tmp/codex-ipc/ipc-<uid>.sock to $CODEX_HOME/ipc/ipc.sock to avoid ownership collisions on multi-user hosts. The TUI must discover the new socket without breaking compatibility with older extension versions.

What changed

• On Unix, try $CODEX_HOME/ipc/ipc.sock first, resolving $CODEX_HOME through the existing helper.
• Fall back to the legacy socket only when primary path resolution, validation, or connection fails.
• Share one timeout budget across both attempts and do not fall back after a successful primary connection encounters a protocol, send, read, or request error.
• Preserve the existing ownership and permission checks and Windows named-pipe behavior.

Manual validation

• Built the modified CLI and configured an isolated VS Code Extension development host to use it with an isolated CODEX_HOME. After selecting a file containing CODEX_IPC_E2E_PRIMARY_PATH_MARKER and enabling /ide, the TUI returned that exact marker through $CODEX_HOME/ipc/ipc.sock. The extension-created IPC directory and socket had modes 0700 and 0600.
• Repeated the IDE-selection check with a second CODEX_HOME that had no primary socket and a controlled legacy <tmp>/codex-ipc/ipc-<uid>.sock proxy. The TUI returned the same exact marker, confirming the legacy fallback path works with the updated extension.

Fixes #22966.