CrossNamespace: Testing Data Plane Changes for Broker Filter

[yijie-04]

Problem
After #7913 is merged, we should add unit tests with BrokerRef for cross-namespace referencing when the CrossNamespaceEventLinks feature flag is enabled. It should be similar to the current tests in pkg/broker/filter/filter_handler_test.go

Persona:
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Exit Criteria
A measurable (binary) test that would indicate that the problem has been resolved.

Time Estimate (optional):
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Additional context (optional)
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