Add code fixer for CA1859

[Copilot]

CA1859 identifies declarations that can use a concrete type but previously required manual edits.

• Code fix
  • Replaces field, property, local, parameter, and return types with the recommended concrete type.
  • Supports Fix All and preserves nullable annotations.
  • Omits fixes for shared multi-variable declarations where changing one type could affect unrelated variables.

IList<string> values = new List<string>();
// becomes
List<string> values = new List<string>();

• Analyzer metadata
  • Carries the target type and nullability information in diagnostic properties.
  • Marks CA1859 as having an available code fix.

• Fixes CA1859 should have an auto-fixer #55747

[azure-pipelines]

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