Add entry to subevent log when filtering out recipient

[MartinRiese]

Product Description

When alert recipients are filtered out it is hard to figure out why. Currently we have bug in production that we cannot reproduce otherwise. This will help determine what exactly is happening.

Technical Summary

https://dimagi.atlassian.net/browse/SUPPORT-22779

Feature Flag

None

Safety Assurance

Automated tests and tested on staging.

Safety story

Automated test coverage

Yes.

QA Plan

Rollback instructions

• This PR can be reverted after deploy with no further considerations

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: corehq/messaging/scheduling/scheduling_partitioned/models.py

Function	Unhandled Issue
send_current_event_content_to_recipients	AttributeError: 'NoneType' object has no attribute 'case_json' corehq.messaging.scheduling.tasks.handle_case_alert_sch... Event Count: 15

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[Jtang-1]

minor comments