fix: chainconfig engine mismatch

[hominlee-wemade]

Summary

This PR fixes an issue where the WBFT consensus engine is initialized using a stale ChainConfig loaded from the database before chain config overrides are applied.

As a result, hardfork settings defined in the genesis or via code-level overrides were not reflected in the consensus engine at initialization time, causing a mismatch between the blockchain and the engine configuration.

This change ensures that the consensus engine is always created using the final ChainConfig with overrides applied.

Background

In the standard Ethereum initialization flow, the final ChainConfig is resolved inside NewBlockChain via SetupGenesisBlockWithOverride.

However, in the current go-stablenet flow:

1. LoadChainConfig loads the ChainConfig from the database
2. The consensus engine is created using this ChainConfig
3. NewBlockChain later applies overrides through SetupGenesisBlockWithOverride, producing the final ChainConfig

This leads to the consensus engine and the blockchain using different ChainConfig values.

Problem

Component	ChainConfig used
Consensus engine	Stored ChainConfig (overrides not applied)
Blockchain	Final ChainConfig returned by SetupGenesisBlockWithOverride

Impact

• Hardfork-related settings are not reflected in the consensus engine
• WBFT engine initialization (e.g. Transitions, SystemContractUpgrades) is based on outdated configuration
• Potential for unexpected consensus behavior or block validation errors due to configuration mismatch

Solution

Key idea

Use a lightweight helper that resolves the final ChainConfig with overrides applied without committing the genesis block, and use it to initialize the consensus engine.

Changes

1. Add LoadChainConfigWithOverride

• Loads ChainConfig from the database or genesis
• Applies overrides
• Uses the same config-resolution logic as SetupGenesisBlockWithOverride

2. Update engine initialization flow (eth/backend.go)

• Build overrides before engine creation
• Replace LoadChainConfig with LoadChainConfigWithOverride
• Create the consensus engine using the override-applied ChainConfig

3. No changes to NewBlockChain signature

• No impact on existing callers or tests