feat: add native gas pre-flight check and update unit tests

[DanFrolov]

Description

This PR introduces a "fail-fast" gas pre-flight check within the ensure_opg_approval process. Currently, if a user has sufficient OPG tokens but lacks native ETH (Base Sepolia) for gas, the transaction fails late with a generic RPC error from the node.

This change ensures that we validate the user's native balance before building or signing the transaction, providing a much clearer error message and saving unnecessary computation.

Key Changes

• Gas Estimation Logic: The process now estimates gas for the approve call and applies a 20% limit buffer to ensure the transaction doesn't run out of gas.
• Pre-flight Balance Validation: Added a check to compare the wallet's native_balance against the total_cost (gas limit * gas price), including a 10% safety buffer to account for potential gas price volatility.
• Enhanced Error Handling: If the balance is insufficient, it raises a RuntimeError that includes:
  • The specific Chain ID.
  • The required ETH vs. available ETH (formatted via Web3.from_wei).
  • A direct instruction to fund the wallet via the Base Sepolia faucet.
• Unit Test Modernization: Updated tests/opg_token_test.py to:
  • Include mock ETH balances in existing tests so they remain compatible.
  • Use realistic transaction dictionaries in build_transaction mocks to satisfy eth-account signing requirements.

Testing Performed

• Mock Verification: Verified that a RuntimeError is raised when the mock balance is 0.
• Success Verification: Verified that the transaction proceeds to signing and submission when the balance is sufficient.
• Regression Check: Ran the full suite of 11 tests to ensure no breakage in OPG balance capping or amount conversion logic.
• Result: 11 passed, 3 warnings in 3.42s.

[kylexqian]

We'll be using OPG as the gas token soon -- so this may not be useful