## 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`
## Summary
- add `ProcessOwnedCodeModeSessionProvider` and logical session
generation/rebinding state
- add the supervised child-process connection, reader/writer tasks, and
driver state machine
- make dropped execute/wait/open callers cancellation-safe with explicit
ownership handoff and durable cleanup
- validate cell/delegate lifecycle state and reject invalid protocol
transitions
- add end-to-end stdio coverage for delegates, cancellation, frame
limits, child loss, stale generations, replacement, and long-lived
sessions
## Why
This final stage exposes the process-owned client only after the wire
protocol, host-safe runtime, and standalone host are independently in
place. Transport failure is fail-stop: the client closes local state,
cancels callbacks, reaps the child, and lazily rebuilds a fresh host
generation rather than transactionally recovering the old connection.
## Stack
This is **4 of 4** in the process-owned code-mode session stack.
- Depends on #30111
- Full stack: #30108 → #30110 → #30111 → this PR
## Validation
- `just test -p codex-code-mode -p codex-code-mode-host` — 86 passed
- `just fix -p codex-code-mode`
- `just fix -p codex-code-mode-host`
- `just bazel-lock-update`
- `just bazel-lock-check`
- `bazel test //codex-rs/code-mode:code-mode-unit-tests
//codex-rs/code-mode-host:code-mode-host-unit-tests
//codex-rs/code-mode-host:code-mode-host-stdio-test
//codex-rs/code-mode-protocol:code-mode-protocol-unit-tests` — 4/4
passed
- `just fmt`
