fix: session idle timestamp correction

[pauldambra]

In https://posthoghelp.zendesk.com/agent/tickets/16379 we see a session that emits zero events for > 10hrs and so doesn't trigger idle detection

when the session returns we correctly flushed only the session idle event and started a new session

but!

the session idle event has as its timestamp the point it was detected making for an 10hr+ session with 10 hours of inactivity

Since we know the threshold and the time of last activity we can correct the timestamp of the event before emitting it

---

tested locally and can see sessionIdle not being emitted at the time it was captured

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Sep 22, 2024 3:12pm

[github-actions]

Size Change: +556 B (+0.05%)

Total Size: 1.22 MB

Filename	Size	Change
dist/array.full.js	349 kB	+139 B (+0.04%)
dist/array.js	164 kB	+139 B (+0.08%)
dist/main.js	165 kB	+139 B (+0.08%)
dist/module.js	164 kB	+139 B (+0.08%)

ℹ️ View Unchanged

Filename	Size
dist/exception-autocapture.js	10.5 kB
dist/recorder-v2.js	111 kB
dist/recorder.js	111 kB
dist/surveys-preview.js	59.8 kB
dist/surveys.js	66 kB
dist/tracing-headers.js	8.36 kB
dist/web-vitals.js	10.3 kB

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