Add code fixer for MA0175 (Remove explicit class keyword from record declarations)

[Copilot]

MA0175 flagged record class declarations but had no code fix to act on the diagnostic. This adds the fixer and corrects a copy-paste error in the rule's documentation.

Changes

• New fixer RecordClassDeclarationShouldBeImplicitFixer: removes the class keyword from record class declarations, preserving surrounding trivia
• Tests: added WithCodeFixProvider to the existing test builder and three fix-validation test cases (basic, with modifiers, with parameters)
• Docs: corrected MA0175 description (was incorrectly describing MA0174's behaviour); documentation tables updated to reflect ✔️ code-fix availability

Example

// Before (MA0175 diagnostic)
public sealed record class Customer(int Id);

// After fix
public sealed record Customer(int Id);