Add [Mutable] code fix for RG0015-RG0018 warnings

[Copilot]

• Analyze the repository structure and understand the analyzer patterns

• Understand RG0015-RG0018 warnings (records immutability checks)

• Understand how the [Mutable] attribute works in RG.Annotations

• Review existing code fix providers for patterns

• Build and test to verify the current state

• Create the AddMutableAttributeCodeFixProvider

• Add tests for the new code fix provider

• Update README.md documentation

• Run code review and address issues

• Run CodeQL security scan - No vulnerabilities found

• Fix indentation to use consistent 4-space indentation in test strings

• Fix newline placement - removed extra trailing newline from attribute so Roslyn Formatter handles spacing correctly

• Preserve original newline trivia between declarations (e.g., blank line between MyClass and RecordName)

• Fixes New code fix: Add [Mutable] to records with RG0015-0018 warnings #55

Original prompt

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This section details on the original issue you should resolve

<issue_title>New code fix: Add [Mutable] to records with RG0015-0018 warnings</issue_title>
<issue_description>Offer this code fix where those warnings appear. The code fix would effectively resolve the warnings</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes New code fix: Add [Mutable] to records with RG0015-0018 warnings #55

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[ronnygunawan]

Fix the indentation

[Copilot]

Fix the indentation

Fixed in 935c96b. The test strings now use consistent 4-space indentation throughout (both input and expected output), matching what the Roslyn Formatter produces when applying the code fix.

[ronnygunawan]

This looks funny. the newline should be before the annotation, not after

[Copilot]

Fixed in a10cca8. Simplified the trivia handling - the attribute now takes the record's original leading trivia without adding an extra trailing newline. This allows the Roslyn Formatter to handle the spacing correctly. The blank line now properly appears before the [Mutable] annotation as the attribute inherits the record's original leading trivia.

[ronnygunawan]

You should respect the original newline trivia between MyClass and RecordName

[Copilot]

Fixed in 60c80ee. The code fix now properly preserves the original blank line trivia between declarations. When there's a blank line before the record (like between MyClass and RecordName), it will now appear before the [Mutable] attribute as expected.