Migrate hardhat tests to Forge

[syntrust]

Addressing #152, this PR translated Hardhat tests into Solidity using Foundry.

Additionally, some cleanups were done, and the test files were separated from the source code. No Solidity source code change.

UT workflow passed.

Note that the environment variable RPC_URL_L1 needs to be set locally (or as a repository secret in GitHub Action) to enable some of the tests, like testCompleteMiningProcess.

[qzhodl]

Since we’ve already deployed the mainnet code, a safer approach might be to split this PR into two smaller ones:

• Forge migration only: Convert the project into a pure Forge setup and update/migrate all tests accordingly. (No functional Solidity changes.)
• Solidity warnings cleanup: Address the Solidity compiler/linter warnings in a follow-up PR, once the Forge migration is merged and stable.

[syntrust]

Since we’ve already deployed the mainnet code, a safer approach might be to split this PR into two smaller ones:

• Forge migration only: Convert the project into a pure Forge setup and update/migrate all tests accordingly. (No functional Solidity changes.)
• Solidity warnings cleanup: Address the Solidity compiler/linter warnings in a follow-up PR, once the Forge migration is merged and stable.

The Solidity code in this PR is all test code; there is no Solidity warnings cleanup.