test: close every remaining coderabbit finding on PR #473

Five new vitest cases plus two refactors that close out the last
batch of CodeRabbit comments on the uninstall PR.

New file: test/codex-manager-uninstall-dispatch.test.ts (F1)
  - "forwards a working clearAccounts handler when --clear-accounts
    is set" exercises the dispatcher branch in lib/codex-manager.ts
    by calling runCodexMultiAuthCli(["uninstall","--clear-accounts"])
    with vi.mock-injected runUninstallCommand and storage.clearAccounts;
    asserts the storage handler is actually invoked. The flag-only
    wiring previously regressed once in this PR's history.
  - "forwards a clearAccounts handler regardless of whether the flag
    is set" pins the dispatcher contract: deps.clearAccounts is
    always present and runUninstallCommand owns the gating.

New file: test/uninstall-ebusy-retry.test.ts (F14)
  - vi.mock("node:fs/promises") with a one-shot EBUSY injector
    forces a single readFile or rm call to throw EBUSY exactly once.
  - Two cases drive runUninstallCommand through its withFileOperationRetry
    loop: one for the Codex.json read path, one for the cache
    rm path. Both assert exit 0 and no "skipped" warnings, proving
    the retry transparently absorbed the transient failure.

test/codex-manager-cli.test.ts (F3)
  - "treats a quota cache save failure as a partial-success warning,
    not a hard failure" rejects saveQuotaCache once with EBUSY after
    account fixes were already persisted, asserts runFix returns 0
    and the JSON payload includes quotaCacheSaveError so callers can
    surface the cache failure without losing the primary fix.

test/preuninstall.test.ts
  - "concurrent invocations leave Codex.json valid and free of
    codex-multi-auth" (F11): runs two runPreuninstallCleanup calls
    in parallel against the same temp home and asserts the resulting
    Codex.json is valid JSON without codex-multi-auth.
  - resolveTempPaths refactor (F10): the local helper that duplicated
    the platform-specific configBase/cacheBase logic is replaced with
    a call to the production resolveInstallPaths so the test cannot
    silently drift from real install/uninstall paths.

test/stream-failover.test.ts (F4)
  - "does not emit an unhandled rejection if releasing the upstream
    reader throws" pins the safety net at lib/request/stream-failover.ts:230.
    Wraps an upstream reader so its releaseLock throws, drives the
    response through withStreamingFailover, asserts the consumer's
    body errors (no hang) and that no unhandled rejection escaped.

Local verification:
- npx tsc --noEmit → clean
- npx vitest run on 8 affected suites → 63/63 (the new files plus
  every previously-touched test file).
- npx vitest run test/codex-manager-cli.test.ts -t "quota cache save
  failure" → 1/1.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>