Review lock usage vs. ConcurrentDictionary in Tca955x interrupt state management

There is a mix of lock-based synchronization and usage of ConcurrentDictionary for interrupt state management in the Tca955x driver. Based on feedback in PR #2427 (see [review comment](https://github.com/dotnet/iot/pull/2427#discussion_r2378814792)), please review all code in the Tca955x driver where locks are used for state management and evaluate if ConcurrentDictionary can be used instead for consistency and thread safety. If lock usage is still required, add rationale as comments in the code.

References:
- https://github.com/dotnet/iot/pull/2427#discussion_r2378814792

Action items:
- Audit interrupt state management code in Tca955x for locks
- Replace locks with ConcurrentDictionary where possible
- Add comments justifying any remaining locks