feat(chain): add ChainState::open() for load-or-create

[Sandipmandal25]

Description and Notes

Closes #911.

ChainState::new() does not check whether the underlying store is already populated. As a result, calling it on an existing store resets the chain to genesis (height 0).

This change introduces ChainState::open(), which:

• Attempts load_chain_state first
• Falls back to new() only if the store is empty (ChainNotInitialized)
• Propagates all other errors unchanged

To keep the API generic over different ChainStore implementations, the method accepts a factory closure instead of a FlatChainStoreConfig. The existing BlockchainError: From<E> bound already covers all DatabaseError implementors via the blanket implementation.

On the caller side:

• florestad::load_chain_state and the load_chain_store helper are replaced with a single ChainState::open() call
• The unused CouldNotCreateFlatChainStore error variant is removed

How to verify

cargo test -p floresta-chain --features flat-chainstore open_resumes_existing_chain_state

stopped at height 16001, restarted, loaded at height 18000.