fix(logs): Handle prototype buffer keys

[scttcper]

Follow-up from #116996 while checking for other plain-object dynamic key indexes.\n\nThis updates the streamed logs timeseries buffer to use own-property checks before reading group values. That keeps group-by values like constructor from resolving to Object.prototype.constructor before the buffer entry exists.\n\nTested with:\n\npnpm test-ci static/app/views/explore/logs/useStreamingTimeseriesResult.spec.tsx

[github-actions]

📊 Type Coverage Diff

Metric	Before	After	Delta
Coverage	93.64%	93.64%	±0%
Typed	132,336	132,336	±0
Untyped	8,986	8,988	🔴 +2

🔍 2 new type safety issues introduced

Non-null assertions (!) (2 new)

File	Line	Detail
static/app/views/explore/logs/useStreamingTimeseriesResult.tsx	275	groupBuffers[groupValue]!
static/app/views/explore/logs/useStreamingTimeseriesResult.tsx	490	groupBuffers[groupValue]!

This is informational only and does not block the PR.

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀