Add namespace to role binding subject

[ShuaiShao93]

This closes #2006

Note to reviewers: remember to look at the commits in this PR and consider if they can be squashed

Summary Of Changes

Additional Context

Local Testing

Please ensure you run the unit, integration and system tests before approving the PR.

To run the unit and integration tests:

$ make unit-tests integration-tests

You will need to target a k8s cluster and have the operator deployed for running the system tests.

For example, for a Kubernetes context named dev-bunny:

$ kubectx dev-bunny
$ make destroy deploy-dev
# wait for operator to be deployed
$ make system-tests

[Zerpet]

No objections to this change.

Please note that since the 28th of July 2025, all external contributors to RabbitMQ will have to sign a CLA again.

You have detailed instructions in [1][2].

Note companies that contribute to RabbitMQ only need to sign the CLA once, there is no need to make every
contributing employee to do so separately.

https://github.com/rabbitmq/cla
https://github.com/rabbitmq/rabbitmq-server/blob/main/CONTRIBUTING.md

[ShuaiShao93]

No objections to this change.

Please note that since the 28th of July 2025, all external contributors to RabbitMQ will have to sign a CLA again.

You have detailed instructions in [1][2].

Note companies that contribute to RabbitMQ only need to sign the CLA once, there is no need to make every contributing employee to do so separately.

https://github.com/rabbitmq/cla https://github.com/rabbitmq/rabbitmq-server/blob/main/CONTRIBUTING.md

I have just signed the CLA. Can I merge this now?