Read persistent subscription filter

[FleaFX]

Is your feature request related to a problem? Please describe.
When working with persistent subscriptions via gRPC, it’s not currently possible to retrieve the original stream or event type filter that was used when creating the subscription. This makes it difficult to introspect or validate the configuration of existing subscriptions — especially in environments where these may have been created dynamically or by other teams/services. In our case, we'd like to inspect the filter to determine whether a persistent subscription is outdated in relation to the deployed code that uses it, so that we can safely delete and recreate it with an updated filter configuration.

Describe the solution you'd like
Expose the filter information (e.g., prefix or regex filters on stream names or event types) as part of the response from the GetInfo or ListTo operations in the gRPC API for persistent subscriptions. Ideally, this would include both the type of filter (stream name or event type) and the matching strategy (prefix, regex, etc.).

Describe alternatives you've considered

• Reading the $persistentSubscriptionConfig stream and parsing the raw JSON events to extract filter details. This works, but it's an implementation detail and not officially supported or documented, which makes it fragile.

• Keeping a separate configuration store outside of KurrentDB — but this leads to duplication and potential drift between declared and actual subscription configurations.

[linear]

DB-801 Read persistent subscription filter