fix: Keycloak organization member association 415 Unsupported Media Type

[sentry]

This PR fixes an HTTPError: 415 Client Error: Unsupported Media Type that occurred when attempting to associate a user with a Keycloak organization.

The root cause was identified as the associate method in b2b/keycloak_admin_api.py sending the user ID (target_id) using the data= parameter in the requests library. This results in the request body being sent as application/x-www-form-urlencoded.

Keycloak's /admin/realms/olapps/organizations/{id}/members endpoint, however, expects the request body to be application/json.

The fix changes data=target_id to json=target_id in the realm_request call within the associate method. This ensures the user ID is sent as a JSON payload, resolving the 415 Unsupported Media Type error.

Fixes MITXONLINE-5YF

[github-actions]

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[pdpinch]

I asked Sentry to open this while I was looking for a different error with enrollments.

To be perfectly clear, I don't know a) how significant this error is nor b) whether this change actually fixes it. Sentry's AI tried to write a test but it somehow ended up deleting tests instead of adding them.

@rhysyngsun I think you looked once at the original sentry issue. Can you give them no more than 5m of your time to determine whether it's worth pursuing?