fix(codex): create CODEX_HOME before pinning it in the app-server env

[dmorosanu]

Problem

Every codex run with mocks configured on POSIX fails at startup since #26:

Failed to initialize Codex client: Codex process closed stdout. stderr_tail=WARNING: proceeding, even though we could not create PATH aliases: CODEX_HOME points to "/root/.codex", but that path does not exist
Error: CODEX_HOME points to "/root/.codex", but that path does not exist

#26 pins CODEX_HOME in the app-server env (so codex state does not follow the redirected login-shell HOME). The codex binary treats an explicitly set CODEX_HOME strictly: it hard-errors when the path does not exist, whereas with the env var unset it materializes the ~/.codex default itself. On runners that auth via CODEX_API_KEY (never ran codex login), ~/.codex does not exist, so the app-server dies at startup. Dev machines have ~/.codex, unit tests pointed CODEX_HOME at existing tmp dirs, and Windows never enters the login-shell path - which is why nothing caught it.

Fix

Create the resolved dir (mkdir(parents=True, exist_ok=True)) before pinning it - mirrors the binary's own default-dir behavior, idempotent across start() retries.

Validation

• New regression test test_build_codex_env_creates_missing_codex_home (fails on main, passes here).
• Reproduced against the bundled codex-cli 0.144.4 binary: codex app-server with CODEX_HOME pointing at a nonexistent dir exits 1 with the exact error above; with the dir pre-created it starts cleanly.
• Full suite: 3342 passed (2 pre-existing Windows symlink-privilege failures in test_sandbox.py, also failing on main); ruff format/check and pyright clean.

[github-actions]

Claude finished @dmorosanu's task in 1m 23s —— View job

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