Reduce redundant site API requests during Atomic wait

[paulopmt1]

Summary

During the entrepreneur setup flow's loading screen (waitForAtomic step), the app aggressively polls GET /rest/v1.2/sites/{id} every 1 second while waiting for the site to become Atomic. This generates 25+ identical requests over ~60 seconds, each returning ~62KB of data — all just to check a single boolean (is_wpcom_atomic).

Additionally, when the processing step mounts, two independent code paths in useSite() fire simultaneous requests for the same site data:

• SITE_STORE resolver → GET /rest/v1.1/sites/{id}?force=wpcom
• Redux requestSite → GET /rest/v1.2/sites/{id}

This PR addresses both issues:

1. Exponential backoff for waitForLatestSiteData

File: client/landing/stepper/hooks/use-wait-for-atomic.ts

Changed from a fixed 1s polling interval to exponential backoff starting at 3s, doubling each iteration, capped at 15s (3s → 6s → 12s → 15s → 15s...). This reduces requests from ~25 to ~6 with negligible UX impact since the user is on a loading screen anyway. This matches the pattern already used by waitForPluginInstall.

2. Deduplicate dual-fetch in useSite()

File: client/landing/stepper/hooks/use-site.ts

Previously, useSite() triggered two independent fetch paths on mount:

1. useSelect → SITE_STORE getSite() resolver → v1.1 API request
2. useEffect → Redux requestSite() → v1.2 API request

Now, the hook waits for the SITE_STORE resolver to complete first. If it succeeds, the site data is synced to the Redux store via receiveSite() — no second network request. If the resolver fails, it falls back to requestSite() as before.

Test plan

• Go through the entrepreneur setup flow end-to-end
• Verify the waitForAtomic loading screen completes successfully
• Monitor network requests — should see ~6 site data requests instead of 25+
• Verify no duplicate simultaneous site requests on processing step mount
• Verify site data is available in Redux store after SITE_STORE resolves

🤖 Generated with Claude Code

[github-actions]

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[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• agents-manager
• help-center
• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug fix/reduce-redundant-site-requests-during-atomic-wait on your sandbox.