refactor: introduce MegaDatabase trait to ensure DB and SaltEnv consistency

[WonderLawrence]

Summary

This PR addresses a correctness issue where ExternalEnvs::salt_env could not guarantee it accessed the same database snapshot as the EVM's Database. The solution introduces a MegaDatabase trait that combines Database + SaltEnv with matching error types, ensuring type-level consistency.

Problem

Previously, SaltEnv was a separate field in ExternalEnvs, which could not guarantee that it accessed the same database snapshot as the EVM's Database. This could lead to correctness issues where SALT bucket capacity queries and database operations might read from different state snapshots.

Solution

Introduce a MegaDatabase trait that requires any database implementation to also implement SaltEnv with matching error types. This ensures at compile time that the database and SALT environment operate on the same underlying data source.

Key Changes

Core Trait Definition

• Add MegaDatabase trait in external/salt.rs with blanket implementation
• Any type implementing both Database and SaltEnv with matching error types automatically implements MegaDatabase

Refactoring

• Remove salt_env field from DynamicGasCost, pass db parameter to methods instead
• Remove salt_env from ExternalEnvs (now only contains oracle_env)
• Update all DB: Database constraints to DB: MegaDatabase throughout the codebase
• Remove unused BlockNumber parameter from ExternalEnvFactory::external_envs()

Supporting Changes

• Add EmptyMegaDB wrapper implementing both Database and SaltEnv for testing
• Implement SaltEnv for test databases (MemoryDatabase, ErrorInjectingDatabase)
• Update documentation to reflect the new architecture

Benefits

1. Type Safety: Compile-time guarantee that Database and SaltEnv are the same instance
2. Backward Compatible: Blanket implementation means downstream users only need to implement both traits
3. Simplified Interface: Removed unused BlockNumber parameter
4. Clear Separation of Concerns: ExternalEnvs now only handles Oracle environment, SALT is provided by the database

Testing

• All existing tests pass with the new trait structure
• Test databases (MemoryDatabase, ErrorInjectingDatabase) properly implement SaltEnv
• EmptyMegaDB provides a working default implementation

[Troublor]

是个不错的思路，不过我不确定在下游组件中好不好改。这个新的trait在mega-reth的哪个分支用上了吗？我想看看mega-reth那边是怎么用的。