Track missing Bizible cookie on lead submissions

[envoy-adam]

Summary

• check for the Bizible _biz_uid cookie when a Marketo form is submitted
• add bizible_cookie_missing to the generate_lead dataLayer event

Validation

• node --check src/utils/form-handlers.js
• git diff --check

[Copilot]

Pull request overview

This PR adds client-side tracking to detect when the Bizible _biz_uid cookie is missing at Marketo form submission time, and includes that signal in the generate_lead GTM/dataLayer event payload to support downstream analytics/attribution monitoring.

Changes:

• Introduces a BIZIBLE_COOKIE_NAME constant and a small hasCookie() helper.
• Computes bizibleCookieMissing on Marketo form submit.
• Adds bizible_cookie_missing to the generate_lead window.dataLayer.push(...) event payload.

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