fix: more compiler warnings

[coffee-the-dev]

PR-Codex overview

This PR focuses on modifying function signatures and return types across several contracts, enhancing the clarity and efficiency of the code.

Detailed summary

• In SessionLib.sol, added a return statement to return 0.
• In MockModule.sol, changed isInited(address account) to isInited(address).
• In ERC20PaymasterMock.sol, changed callOracle to external pure.
• In GaslessPaymaster.sol, initialized context to an empty string.
• In SessionKeyValidator.sol, modified handleValidation to exclude the signature parameter.

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