OUT-3544: Add retry mechanisms for transient network errors

[SandipBajracharya]

Summary

• Expand withRetry classifier to cover transient 5xx (500/502/503/504), undici network failures (ECONNRESET/ECONNREFUSED/ETIMEDOUT/UND_ERR_CONNECT_TIMEOUT), and AbortSignal.timeout/AbortController rejections. Tune factor: 4, retries: 4 so maxTimeout: 10_000 actually engages on the final attempt (back-off becomes 0.5s/2s/8s/10s ≈ 20.5s across 5 attempts).
• Add AbortSignal.timeout(EXTERNAL_FETCH_TIMEOUT_MS) (default 30s, env-overridable) to every external fetch in fetch.helper.ts and copilotAPI.manualFetch.
• Validate response.ok in fetch helpers; non-2xx now throws a typed HttpFetchError (status, statusText, url, body — JSON-parsed if possible, raw text otherwise) instead of cryptic JSON-parse failures.
• Remove 15 dead if (!res) throw new APIError(... 'no response') guards from intuitAPI.ts (97 LOC) now that helpers throw on non-2xx; route real status through getMessageAndCodeFromError so qb_sync_logs records 503 as 503 instead of 500.
• Update useSettings/useQuickbooks to wrap postFetcher calls in try/catch since helpers now throw.

Worst-case op duration: 5 × 30s + 20.5s ≈ 170s, well within the webhook route's maxDuration = 300.

Test plan

• yarn vitest run --project unit (72/72 passing — adds 19 cases covering 5xx retries, undici/cause network errors, TimeoutError, fetch.helper response.ok and timeout paths)
• yarn lint:check clean (only pre-existing warnings)
• yarn prettier:check clean
• yarn tsc --noEmit clean
• Manual smoke against QBO sandbox: trigger a webhook, point one outbound at an unroutable host to force ECONNREFUSED, confirm 5 attempts in qb_sync_logs over ~20s before final failure
• UI spot-check: product/invoice settings save flow still surfaces error state correctly when the API route returns non-2xx

🤖 Generated with Claude Code

[linear-code]

OUT-3544 Add retry mechanisms for transient network errors

[vercel]

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Project	Deployment	Actions	Updated (UTC)
quickbooks-sync	Building		May 6, 2026 5:05pm
quickbooks-sync (dev)	Ready	Preview, Comment	May 6, 2026 5:05pm