Fix remote TUI cwd decoding across OSes

[etraut-openai]

Fixes #21357.

Why

Native Windows TUI sessions connected to a remote app-server running on another OS can fail during thread lifecycle bootstrap because v2 thread lifecycle responses carry server filesystem paths in AbsolutePathBuf fields. A Linux app-server can return paths such as /home/..., which are not local absolute paths on Windows, so typed deserialization fails before the TUI can finish thread/start.

What changed

• Route TUI thread/start, thread/resume, and thread/fork responses through a shared lifecycle helper that captures the raw server cwd and deserializes with a local AbsolutePathBufGuard.
• Preserve the server-native cwd for later remote turn/start requests so the TUI does not send locally decoded placeholder paths back to the app-server.
• Add regression coverage for lifecycle responses whose path strings need a local base to deserialize.

Verification

• cargo test -p codex-tui app_server_session
• just fix -p codex-tui

[chatgpt-codex-connector]

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[etraut-openai]

@codex review

[fcoury-oai]

I tested this from Windows against a Linux remote app-server. It gets further than before, but I think the PR still doesn't fully fix the issue yet.

The TUI starts, but then skills/list fails with:

AbsolutePathBuf deserialized without a base path

And submitting a prompt fails at turn/start.

So this seems to fix part of the lifecycle path, but not the full cross-OS remote TUI flow described by the title/body.

I think we either need to expand the fix to the other remote app-server responses that carry server paths, or narrow the PR scope/title.