fix(dsh-plugin): bound output draining after child exit

[iuyo5678]

When a bsk child exits without emitting close, the plugin keeps waiting even after its command timeout: cancellation sees an exited process and cannot settle the pending run. A successfully created browser session can consequently remain absent from the plugin registry.

Allow one second to drain trailing stdout/stderr after exit, then release the remaining pipes and complete with the observed exit code and cancellation state. Normal close still completes immediately, and the existing cooperative cancellation and force-kill grace periods are preserved. Shared settlement cleanup clears all timers and prevents late events from completing a run twice.

Validation:

• Added 14 lifecycle regressions across the Windows and POSIX code paths; 10 failed against main before the fix.
• All 254 plugin tests, TypeScript checks, formatting checks, and the production build/pack passed.
• Windows 11, Node 24.19.0, dsh 0.1.5-rc.2, bsk/daemon/Edge extension 0.3.0: the installed fixed package passed 5/5 normal session start/list/stop cycles.
• In a real ConPTY dsh host, deliberately withholding the plugin children's close events still allowed 5/5 complete tool-pipeline start/list/stop cycles (1.40–1.65 seconds for start). Under the same injected condition, the unmodified npm package remained pending and its session list was empty.
• A short-timeout fault-injection case now returns its timeout error. Test-created browser sessions and temporary dsh hosts were cleaned up.

Refs #180. This fixes the confirmed missing-close completion gap; it does not claim that Windows or ConPTY naturally loses the event. Scope is limited to the runner and its regression tests.