[APPS-2792] Add: local-execution resilience tests (Milestone 7)

[tyffical]

Motivation

• Milestone 7 in the Local Node Execution Kickoff doc — renamed/descoped from "Chaos-engineering resilience testing" since domains/chaos-engineering targets Datadog-owned Kubernetes clusters, a mismatch for this in-process, customer-dev-server model.
• Two targeted resilience checks, not fixes: empirically confirm the actual failure modes of the RFC's "no process isolation" decision, rather than leave them as assumed-but-unverified.

Changes

What changed	File
New test confirming a synchronous CPU-bound loop starves the event loop, so the current Promise.race timeout never gets a chance to fire — it can only settle once the loop finishes on its own.	local-execution.resilience.test.ts
New test (via a real child process, since process.exit() can't safely run inside this same Jest process) confirming process.exit() inside the customer function terminates the whole process immediately, bypassing try/finally cleanup entirely.	local-execution.resilience.test.ts

The third Milestone 7 task ("the queue survives a rejected execution and keeps running") was already covered by an existing test in local-execution.test.ts, per the Kickoff doc's own checklist.

QA Instructions

yarn workspace @dd/tests test:unit packages/plugins/apps/src/vite/local-execution.resilience.test.ts
# Expected: 2 passed ✅ VERIFIED

yarn workspace @dd/tests test:unit packages/plugins/apps
# Expected: 25 suites / 324 tests passed ✅ VERIFIED

yarn workspace @dd/apps-plugin run typecheck
# Expected: clean exit ✅ VERIFIED

Blast Radius

• Test-only change, no production code touched. Zero effect on shipping behavior.
• Risk: none.

Out of Scope / Follow-ups

Item	Status	Next step
Whether to pursue real process/thread isolation (e.g. pooled worker_threads) to close the confirmed sync-hang and process.exit() gaps	deferred	These tests exist specifically to inform that decision with real data rather than speculation — a follow-up design discussion, not blocking this PR.

Documentation

• RFC: Local Node execution for High Code Apps backend functions — "Run in-process instead of forking an isolated child process" trade-off this milestone verifies.
• QA guide: Confluence page 7100498075 — exact local/staging QA commands for this repo

[datadog-prod-us1-6]

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: a0d01dd | Docs | View more details | Give us feedback!}