[MKT-856]:feat/use partnerId instead of utmMedium in checkout

[jaaaaavier]

Description

Context:
Affiliate links from Impact were appending the partner ID to utm_medium (e.g. utm_medium=312695), which broke GA4's channel grouping. The Impact Partner Tracking Template is being updated to use utm_medium=affiliate and utm_content={irpid} instead.

Changes in impact.service.ts

• Renamed the utmMedium param of handleImpactDTCCheckout to partnerId to reflect what it actually represents. The payload sent to Impact (partner_id) is unchanged.
• Added resolvePartnerIdFromUrl() helper that reads the partner ID from the URL with backward compatibility:

Related Issues

Related Pull Requests

Checklist

• Changes have been tested locally.
• Unit tests have been written or updated as necessary.
• The code adheres to the repository's coding standards.
• Relevant documentation has been added or updated.
• No new warnings or errors have been introduced.
• SonarCloud issues have been reviewed and addressed.
• QA Passed

Testing Process

Additional Notes

[cloudflare-workers-and-pages]

Deploying drive-web with    Cloudflare Pages

Latest commit:	c5c2172
Status:	✅  Deploy successful!
Preview URL:	https://44ff7781.drive-web.pages.dev
Branch Preview URL:	https://feat-add-resolve-partner-id.drive-web.pages.dev

View logs

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
90.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud