[Enhancement] Validation of subscription names in the Pulsar broker

### Search before asking

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

### Motivation

Currently, for resources like tenants, namespaces, topics the Pulsar broker will do some validation on the naming to ensure a certain format: https://github.com/apache/pulsar/blob/master/pulsar-common/src/main/java/org/apache/pulsar/common/naming/NamedEntity.java

For subscription names however, there is no validation at all, possibly causing issues in downstream systems.

### Solution

Introduce a validation for subscription names, similar to what is already done with `NamedEntity`.

Because this might break things for users currently using such names, it should be optional first so users have time to make the necessary changes.

### Alternatives

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by reading pulsar-common/src/main/java/org/apache/pulsar/common/naming/NamedEntity.java to understand the existing naming validation. Trace where subscription names enter the Pulsar broker, then determine how validation can be introduced as an optional behavior; done means invalid subscription names are handled consistently without immediately breaking existing users.

Written by the indexing model from the issue text.