refactor: simplify index retrieval in InstanceCounterAnonymizer

[RonShakutai]

Change Description

Refactor the pseudonymization sample notebook to simplify how we compute the next index for a given entity type.
Removed the manual string-parsing logic in _get_last_index and replaced it with a direct return len(entity_mapping_for_type) call for clarity and performance.

Checklist

• I have reviewed the [contribution guidelines](https://github.com/microsoft/presidio/blob/main/CONTRIBUTING.md)
• I have signed the CLA (if required)
• My code includes unit tests
• All unit tests and lint checks pass locally
• My PR contains documentation updates / additions if required

[omri374]

Thanks!