test: pin the insights no-cold-bootstrap contract at the wire, and the persist guard at its callers

[atomantic]

Summary

Follow-up to #5626 — the two review threads we didn't hold that PR open for. Test-only; no production code changes.

• Read-path assertion back at the wire. fix: dedupe insightsService's transport onto the shared aiProvider one #5626 moved the no-cold-bootstrap assertions from a fetchWithTimeout-level spy down to a callProviderAISimple-level one, because the transport moved to aiProvider. That's one export, but the AGENTS.md contract is broader — "PortOS must never queue up AI provider calls a user hasn't knowingly triggered", by any route. A future read path reaching a provider through a direct fetch, a streaming helper, or a second transport would slip past the export-level spy while it stayed green. Restores a global fetch spy alongside it.
• The actual [module-hygiene-insights-api-transport] Reuse aiProvider for Insights LLM calls #5617 bug is now pinned at the Insights boundary. Both generators persist unconditionally once result.error is unset, so a transport that misclassifies a failed call as a successful empty completion overwrites a real cached theme/narrative with nothing. Until now that was only proven transitively — the shared transport returns an error, and you read the if (result.error) return guard at each call site. Now asserted directly: an errored transport result surfaces as { available: false, reason } and leaves atomicWrite untouched, for both generateThemeAnalysis and refreshCrossDomainNarrative — plus the positive case, so the guard can't pass by never writing at all.

Test plan

• cd server && npx vitest run services/insightsService.test.js services/aiProvider.test.js — 39 pass
• Bypass probe (the guard tests are not vacuous): neutering both if (result.error) guards in insightsService.js fails exactly the two persist-guard tests and nothing else; restoring them goes green again
• fileUtils is partial-mocked via importOriginal — PATHS is read at insightsService module scope, so a mock omitting it breaks the import outright