feat: Add EHR export event tracking (M2-9454)

[farmerpaul]

📝 Description

🔗 Jira Ticket M2-9454

Adds these custom properties to the existing Export Data Successful Mixpanel event when an EHR.zip file is successfully downloaded to the user's computer:

• Feature: ["EHR"]
• EHR File Size: <size of EHR.zip in bytes>

📸 Screenshots

🪤 Peer Testing

Preconditions:

• An activity where you have previously ingested EHRs.

Steps:

1. From the Admin App, open the Export modal from any context where EHRs should be included, choose Responses & EHR Data from the dropdown, then click Download.
2. Log into Mixpanel and open the Mindlogger Analytics Dev project. Check the most recent events (refresh after a minute or so) and locate the [Admin] Export Data Successful event. Expand the Your Properties tab.
   Expected outcome: The properties Feature: ["EHR"] and EHR File Size: <size of EHR.zip in bytes> should be present.
3. From the Admin App, open the Export modal from any context where there should be no EHR data, choose Responses & EHR Data from the dropdown (or choose Responses only), then click Download.
4. Check the most recent [Admin] Export Data Successful event in Mixpanel.
   Expected outcome: The properties Feature: ["EHR"] and EHR File Size: <size of EHR.zip in bytes> should not be present.

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