[codex] Preserve child process termination context

[juliusmarminge]

Summary

• attach the child process PID to ACP and Codex App Server termination errors
• retain the exact exit-status read failure as the transport error cause
• keep normal process exits cause-free while preserving their exit code
• cover both symmetric adapters with compact focused tests

Why

Termination failures previously retained the exit code or read failure but not the process identity, which made concurrent child-process failures harder to correlate. The existing termination helper remains because it performs an effectful status read and selects between distinct error types; it is not a constructor-only wrapper.

Validation

• vp test across all effect-acp and effect-codex-app-server test files: 47 passed
• vp check
• vp run typecheck

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Note

Low Risk
Additive optional fields on error types and clearer diagnostics; no changes to auth, data handling, or core transport behavior beyond enriched error payloads.

Overview
Child process termination errors now carry the process ID in both effect-acp and effect-codex-app-server. makeTerminationError passes handle.pid into AcpProcessExitedError / CodexAppServerProcessExitedError on normal exit and into the corresponding transport error when reading exit status fails; the exit-status failure still uses the original cause unchanged.

The termination helper now accepts a narrowed ChildProcessTerminationHandle (pid + exitCode only). Error schemas gain an optional pid field on process-exited and read-exit-status transport types. New stdio.test.ts files assert PID retention, cause identity, and that user-facing messages stay free of nested diagnostic text.

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Note

Preserve child process PID in termination errors for ACP and Codex App Server

• Adds an optional pid field to AcpProcessExitedError, AcpTransportError, CodexAppServerProcessExitedError, and CodexAppServerTransportError schema classes.
• Updates makeTerminationError in both packages to accept a ChildProcessTerminationHandle (typed as Pick<ChildProcessHandle, 'exitCode' | 'pid'>) and populate pid on all produced errors.
• Adds test suites in both packages verifying PID propagation and correct error messages for both exit-code success and failure cases.

^{Macroscope summarized f751e75.}

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[macroscopeapp]

Approvability

Verdict: Approved

This PR adds optional pid fields to error classes and populates them when creating termination errors - a simple, backwards-compatible enhancement for debugging context with no runtime behavior changes.

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