This commit introduces the core provider state and network mapping required for wallet management, focusing on active wallet ownership and lifecycle handling.

Key changes:

Implement active wallet ownership and lifecycle handling in wallet_providers.dart.
Add a self-dispose guard in the provider when set() receives the same wallet instance.
Ensure safe disposal behavior on both clear() and provider disposal.
Introduce a service-layer network mapper in wallet_network_mapper.dart.
Testing:

Add unit tests for ActiveWalletNotifier and WalletNetworkMapper.
Include clear-path lifecycle coverage, verifying that clear() properly disposes of the wallet and resets the state to null.
Verified that flutter analyze and flutter test pass successfully.