fix(project-install): Handle prototype provider slugs

[scttcper]

Follow-up from #116996 while checking for other plain-object dynamic key indexes.\n\nThis updates messaging integration grouping to use own-property checks before reading provider-slug buckets. That keeps provider slugs like constructor from resolving to Object.prototype.constructor before the bucket exists.\n\nTested with:\n\npnpm test-ci static/app/views/projectInstall/issueAlertNotificationOptions.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	8,987	🔴 +1

🔍 1 new type safety issue introduced

Non-null assertions (!) (1 new)

File	Line	Detail
static/app/views/projectInstall/issueAlertNotificationOptions.tsx	113	map[providerSlug]!

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 🥀