Fix cross-site build errors by filtering allowed sites for static params

[sc-shakyawijerathne]

Description / Motivation

This PR fixes a critical build error that occurred during Vercel deployments when multiple XM Cloud sites share the same instance.

Root Cause

During static site generation (SSG), the generateStaticParams function was attempting to prerender pages for ALL sites in the XM Cloud instance, not just the site being deployed. For example, when deploying the SYNC site, it would incorrectly attempt to prerender pages from the Alaris site, causing build failures.

Solution

Modified the generateStaticParams function across all example starters to filter sites based on the defaultSite configuration in sitecore.config. The implementation now:

• Checks if a defaultSite is configured
• Filters the sites array to only include the configured site
• Falls back to all sites if no defaultSite is specified

This ensures each starter only generates static params for pages that belong to its designated site, preventing cross-site build errors when multiple starters share the same XM Cloud instance.

Testing Details

• Unit Test Added
• Manual Test/Other

Testing Performed:

Verified successful Vercel deployments for individual starters in a shared XM Cloud environment
Confirmed static params generation only includes pages from the configured defaultSite