Remove underscore from field name to comply with naming conventions

[Copilot]

Addresses review feedback on #7258 to remove underscore from public static readonly field name.

Changes:

• Renamed TestClassShouldHaveTestMethodRule_BaseClassHasAssemblyAttributes to TestClassShouldHaveTestMethodRuleBaseClassHasAssemblyAttributes
• Updated all references in analyzer and test code

Aligns with .editorconfig rule requiring PascalCase for static readonly fields without underscores.

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