fix: CI failures for rustfmt and clippy

[FabLrc]

This pull request mainly refactors code for improved readability and consistency across several modules, especially in the CheckRunner and GithubClient implementations. The changes focus on formatting, simplifying expressions, and minor logic improvements, without altering core functionality.

Minor Logic Improvements

• Replaced manual string prefix checks with strip_prefix in the is_conventional_commit function for better clarity and correctness.
• Removed unnecessary blank lines and improved grouping of related code blocks.

Consistency in Function Signatures

• Updated function signatures for failed and warning in CheckResult to use single-line formatting, improving consistency and reducing vertical space.

Simplification of Data Processing

• Simplified the process of cleaning and decoding base64 strings in GithubClient::fetch_file_content by using a more concise method to remove unwanted characters.

Minor Refactoring in Check Engine

• Reformatted the closure in CheckEngine for filtering passed and warning statuses, and improved formatting for the date string assignment.