chore: improve the way we supply b2b cdn url

[bc-victor]

What/Why?

• Add BUYER_PORTAL_ENVIRONMENT to select the Buyer Portal CDN: production (default), staging, or integration.
• In ScriptProduction resolve CDN via a simple env→URL map
• Update loader.ts to pass the optional BUYER_PORTAL_ENVIRONMENT (defaults to production).

Testing

• With LOCAL_BUYER_PORTAL_HOST set, dev mode should still load the local app (no changes needed).
• With LOCAL_BUYER_PORTAL_HOST unset:
  • BUYER_PORTAL_ENVIRONMENT=production → https://microapps.bigcommerce.com/b2b-buyer-portal/headless.js
  • BUYER_PORTAL_ENVIRONMENT=staging → https://microapps.staging.zone/b2b-buyer-portal/headless.js
  • BUYER_PORTAL_ENVIRONMENT=integration → https://microapps.integration.zone/b2b-buyer-portal/headless.js
• Verify in the Network tab and ensure the script tag includes data-environment with the expected value.
• Sign in and confirm the Buyer Portal loads and session behaviors (cart sync, events) are unchanged.

Screen.Recording.2025-10-22.at.2.53.24.p.m.mov

Migration

• Optional: replace previous staging/custom origin envs with the new single BUYER_PORTAL_ENVIRONMENT.
  • Remove any use of STAGING_B2B_CDN_ORIGIN if present.
  • Add BUYER_PORTAL_ENVIRONMENT=staging|integration as needed; omit for production (default).
• No code changes required for consumers; B2BLoader now passes environment to ScriptProduction.

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