test(exec): allow loaded runners to observe background exit

[jonathanKingston]

Problem

The background-process manager tests poll for a short-lived child to exit. The helper's five-second deadlock guard can expire during the full parallel suite when a busy runner delays scheduling the new Node child, even though the process and manager are working correctly. The test passes in isolation.

Fix

• Raise only the test helper's deadlock guard from 5 seconds to 30 seconds.
• Keep the 20 ms polling interval, so ordinary successful runs still finish immediately.
• Document why the larger outer guard is necessary under full-suite load.

No product timeout or background-process behavior changes.

Validation

• pnpm test -- src/main/services/exec/background-process.test.ts (19/19)
• pnpm run check (8,543/8,543 tests; typecheck, lint, format, site, dead-code, oracle, and E2E syntax checks all green)

[jonathanKingston]

Validation follow-up (2026-09-05): the focused background-process suite passed 19/19 and the complete local pnpm run check passed 8,543/8,543 under the same parallel load that exposed the five-second guard. GitHub CI is also fully green. This changes only the test helper's deadlock guard; successful polling and product timeouts remain unchanged. No further changes needed.

[github-actions]

🖥️ PR preview

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