Add rekt tests for DefaultBrokerClass config

**Problem**
We need to add rekt (reconciler tests) for the DefaultBrokerClass configuration. The focus should be on validating that the default configuration works correctly with the recent changes.

**Persona**
[Eventing Contributor](https://github.com/knative/eventing/blob/main/docs/personas.md#eventing-contributor)

**Exit Criteria**
- [ ] Rekt tests for DefaultBrokerClass configuration are implemented
- [ ] Tests pass, confirming that the default configuration works as expected with new changes

**Scope**
- This test should focus on the default configuration
- Extensive corner cases are not required, as they will be covered by unit tests

**Time Estimate**
1 developer-day

**Dependencies**
- Blocked by: https://github.com/knative/eventing/pull/7631 (needs to be merged first)

**Additional Context**
- The rekt tests implemented here should be focused and not cover many different corner cases, as those will be handled by unit tests to avoid expensive rekt tests.
- Once PR #7631 is merged, this task will be unblocked and ready for implementation.

First check whether PR #7631 has merged, since this issue is blocked by it. Then locate the existing rekt test entry points for DefaultBrokerClass and add focused coverage for the default configuration; done means the tests pass and confirm the default behavior after the referenced changes.

Written by the indexing model from the issue text.