Add naming conventions for primary constructor parameters

[Copilot]

This PR adds comprehensive naming conventions for primary constructor parameters to the C# identifier naming documentation, addressing a gap identified in issue #42295.

Changes Made

Documentation Updates:

• Added new "Primary constructor parameters" subsection to the "Camel case" section in identifier-names.md
• Clarified the distinction between naming conventions for different type declarations:
  • Class and struct types: Use camelCase (consistent with method parameters)
  • Record types: Use PascalCase (as parameters become public properties)

Code Examples:

• Created new code snippets demonstrating correct naming patterns:

  // Class primary constructor - camelCase
  public class DataService(IWorkerQueue workerQueue, ILogger logger)
  
  // Struct primary constructor - camelCase  
  public struct Point(double x, double y)
  
  // Record primary constructor - PascalCase
  public record Person(string FirstName, string LastName);

Validation:

• All code examples compile and run successfully
• Existing repository examples already follow the documented conventions
• Documentation passes markdownlint validation

This addresses the original question about whether to use Entity(Dependency dependency) vs Entity(Dependency _dependency) - the answer is the former (camelCase without underscore prefix) for regular classes and structs.

Fixes #42295.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/fundamentals/coding-style/identifier-names.md	C# identifier naming rules and conventions

[BillWagner]

This is ready for final review, once my suggestions are merged.