code-mode: fall back to using in process v8 if we fail to resolve external process

[cconger]

Why

Not every Codex distribution currently includes the codex-code-mode-host companion binary. Enabling the process-host feature should not make code mode unavailable on those surfaces while packaging support is being completed.

What changed

• Fall back to an in-process code-mode session only when spawning the companion binary returns io::ErrorKind::NotFound.
• Keep permission, handshake, timeout, and other host failures visible instead of silently falling back.
• Store the provider's owned-process/in-process choice as one enum-backed state so later sessions reuse the fallback decision.
• Preserve the underlying spawn io::Error while retaining the host path in the displayed error.
• Update provider, CodeModeService, and end-to-end coverage to verify successful fallback execution.

Test plan

• just test -p codex-code-mode
• just test -p codex-core missing_process_host

[chatgpt-codex-connector]

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