Missing Test Coverage: Add embedded test for Proxy's PulsarHandlers

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Motivation

The `ServerCnxTest` class is extremely valuable and makes it easy to verify certain low level behaviors. We do not have the same tests for the proxy.

### Solution

Add these tests.

### Alternatives

_No response_

### Anything else?

I am happy to implement this at some point, but for now, I am unable to.

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start by reading the existing ServerCnxTest class and locating the proxy's PulsarHandlers implementation and related embedded proxy tests. Identify the low-level behaviors already covered for ServerCnx, then add equivalent proxy coverage. Done means the embedded tests run successfully and cover the relevant PulsarHandlers behaviors.

Written by the indexing model from the issue text.