Fix invalid cast for x-ms-paths required arrays

[Jerry-Johnson-3]

Summary

• Stop casting a LINQ Select projection to ISet<string> when converting required arrays on x-ms-paths schemas.
• Add a regression test that converts a schema with required: [name].

That cast throws InvalidCastException whenever a custom path schema includes required.

Spec

required is a JSON Schema array of unique strings (OAS Schema Object takes this keyword unchanged). The crash is a C# invalid cast: Enumerable.Select returns IEnumerable<string>, not ISet<string>.

• OAS 3.0.3 Schema Object (required taken from JSON Schema)
• JSON Schema Wright Draft 00 §5.4.3 (required MUST be an array of unique strings)
• Enumerable.Select

x-ms-paths itself is an AutoRest specification extension, not an OAS keyword; the required array on schemas inside it is still JSON Schema required.

Testing

• Red: ToOpenApiPaths_ConvertsSchemaRequiredArray threw InvalidCastException.
• Green: the same test passed after building a HashSet<string>.