test: make resolveClientCommand codex/claude tests hermetic (#642)

[ranxianglei]

Fixes #642

Problem

tests/launcher.test.ts:1064 — resolveClientCommand: codex/claude resolve to themselves hard-coded /usr/bin as a PATH that does NOT contain codex/claude. On any machine with those CLIs in /usr/bin the not-on-PATH assertion fails:

AssertionError: Expected values to be strictly deep-equal:
  {
+   command: '/usr/bin/codex',
-   command: 'codex',
  }

Reproduced in this sandbox (has /usr/bin/codex + /usr/bin/claude): 1 failure before the fix, 0 after.

Change

Split the test into two hermetic tests, following the existing pi / codebuddy (#641) pattern:

• not on PATH: PATH points at an empty temp dir → bare-name fallback (codex / claude)
• on PATH: fake codex / claude executables created in a temp bin dir → the absolute path is returned

Test-only change; src/launcher.ts untouched (resolveOnPath was already correct).

Pre-flight

• npm run typecheck — clean
• npm test — 1241/1241 pass (on this sandbox, where the old test failed)
• npm run build — success

[github-actions]

📦 Built Package Artifact

Branch: 2026-09-08_hermetic-codex-claude-tests (f0a85ec)

Option A — Install from npm PR tag (recommended)

npm install -g billion-context@pr-643

Each push to this PR publishes a new version under the pr-643 npm tag.

Option B — Download artifact

1. Download the artifact from the Actions run
2. Extract the tarball and install:

tar xzf billion-context-pr643.tgz
npm install -g package

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This comment is automatically updated on each push.

[ranxianglei]

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[bot] 🏷 Review: ✅ approved — ready to merge

I did an independent pass rather than taking the pre-flight at face value.

1. Bug is real & reproduced. In this environment /usr/bin/codex and /usr/bin/claude are present, so the old test's PATH: "/usr/bin" assumption ("a PATH that does NOT contain codex/claude") is false. Feeding that assertion through the actual resolveClientCommand returns the full path, not the bare name → the old test fails here. Confirmed pre-existing on master, not PR-introduced.

2. Fix is correct. resolveClientCommand (src/launcher.ts:1639) does resolved ?? client for codex/claude, and resolveOnPath (src/launcher.ts:1620) returns the full path only when the file actually exists. So:

• empty temp-dir PATH → not found → bare-name fallback ✓
• temp bin dir with fake codex/claude → found → absolute path ✓

3. Hermetic & consistent. Both new tests use fs.mkdtempSync(os.tmpdir()/bili-path-*) + a fake executable + try/finally cleanup — a structural match to the existing pi on PATH resolves to full path test and the #641 codebuddy pattern. No reliance on the host's real PATH.

4. Test-only. src/launcher.ts untouched; resolveOnPath was already correct. No product behavior change.

What I ran:

• resolveClientCommand tests: 5/5 pass
• full tests/launcher.test.ts: 103/103 pass
• npm run typecheck: clean
• (build unaffected — test-only; dist bundles src/ only)

Minor nit (non-blocking): in the "on PATH" test the two fs.writeFileSync calls sit before the try, so a write failure would leak the temp dir. This is identical to the existing pi test and writeFileSync into os.tmpdir() is effectively infallible, so I'd leave it as-is for pattern consistency.

Fixes #642 linkage is correct — #642 is exactly this non-hermetic-test report.

LGTM. Merge is human-only per AGENTS.md — please merge when ready: #643