chore: add tests to executable

[Peponks9]

unit tests for the payload execution module

Closes #29

[Copilot]

Pull Request Overview

This PR adds unit tests for the payload execution module (src/payload/exec.rs) and introduces test utility functions for creating reverting and invalid transactions. The changes support testing the Executable enum and its execution methods.

• Adds 23 unit tests covering Executable, ExecutionResult, and IntoExecutable trait implementations
• Introduces reverting_tx and invalid_tx helper functions in test utilities
• Tests cover transaction execution, bundle execution, and various conversion methods

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/test_utils/transactions.rs	Adds reverting_tx and invalid_tx helper functions for creating test transactions with specific failure behaviors
src/payload/exec.rs	Adds comprehensive test suite with 23 tests covering execution of transactions, bundles, and various conversion paths

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[Peponks9]

thanks for the feeback @julio4, looking forward to keep contributing to flashbots :)