Relax rules in `EventSummary` DTO

[devraj]

Describe the bug
While working with a client's Command Centre instance I found that a number of EventSummary (while tailing the /events/updates endpoint)objects were failing to parse due to missing attributes or the attributes being too strict.

To Reproduce
Steps to reproduce the behavior:

1. Run the test_events.py
2. Or try and validate the response from /api/events/updates using EventSummary

Expected behavior
The validation should pass, instead the following fail:

• operator.href required and should be optional
• card.id is not sent down in the response
• modified_item.href is required and it not sent down
• source.href should be optional

Screenshots
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Desktop (please complete the following information):
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Smartphone (please complete the following information):
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Additional context

• See also Support retrieving partial responses from endpoints (refactor for pydantic 2.10) #20
• Command Centre was running version 8.9
• Before we go down debugging this, can we please make sure that the follow command is targeting the right URLs.