Ignore primary constructors in MA0196 and reuse shared detection

[meziantou]

MA0196 should not flag <inheritdoc /> attached to primary constructor syntax, because that XML documentation primarily describes the containing type. This change fixes that false positive and consolidates primary-constructor detection logic.

What changed

• Skip MA0196 diagnostics when the analyzed symbol is a primary constructor.
• Add a shared IsPrimaryConstructor helper in SymbolExtensions.
• Reuse the shared helper in existing primary-constructor detection paths:
  • InheritdocShouldBeUsedOnInheritingMemberAnalyzer
  • PrimaryConstructorParameterShouldBeReadOnlyAnalyzer
  • LocalVariablesShouldNotHideSymbolsAnalyzer
• Add regression tests for MA0196, including a C# 12 primary-constructor case.
• Update MA0196 documentation to explicitly state that primary constructors are ignored.

Notes

• MA0196 uses includeRecordDeclarations: true in the shared helper to cover record primary constructors.
• Targeted analyzer tests were run on default Roslyn plus roslyn4.2 and roslyn4.14.

Fix #1139