Fix MA0148/MA0149 false positives with implicit conversions

[meziantou]

Summary

MA0148 and MA0149 could suggest replacing == or != with is patterns when the comparison only worked through an implicit user-defined conversion. In those cases, the suggested is/is not rewrite is invalid C#.

This change makes the analyzer and fixer skip those cases so diagnostics are only reported when the pattern-matching rewrite is valid.

What changed

• Added HasImplicitUserDefinedConversion in UsePatternMatchingForEqualityComparisonsCommon.
• Updated the MA0148/MA0149 analyzer path to skip constant-comparison diagnostics when the non-constant operand depends on an implicit user-defined conversion.
• Updated the fixer to apply the same guard for single comparisons and logical merge candidates.
• Added regression tests for implicit-conversion equality and inequality cases, plus a mixed-expression case to ensure only valid expressions are rewritten.
• Updated docs/Rules/MA0148.md and docs/Rules/MA0149.md with explicit examples of ignored implicit-conversion scenarios.

Validation

• dotnet test tests/Meziantou.Analyzer.Test --filter "FullyQualifiedName~UsePatternMatchingForEqualityComparisonsAnalyzerTests"
• dotnet test tests/Meziantou.Analyzer.Test /p:RoslynVersion=roslyn4.2 --filter "FullyQualifiedName~UsePatternMatchingForEqualityComparisonsAnalyzerTests"
• dotnet build Meziantou.Analyzer.slnx
• dotnet run --project src/DocumentationGenerator