[BUG][CSHARP-NETCORE] OneOf generator for not nullable primitives generates invalid code.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request

Description

A template that contains oneOf logic for a not nullable primitives generates invalid code.

openapi-generator version

5.4 and ff7c2bdb591dd8fb6dd1c39dbe88367dbea4042d

OpenAPI declaration file content or url

Part of schema relating to this bug report:

PolymorphicProperty:
  oneOf:
  - type: boolean
  - type: string
  - type: object
  - type: array
     items:
  $ref: '#/components/schemas/StringArrayItem'
StringArrayItem:
  type: string
  format: string			

Full schema: https://gist.github.com/jafin/5b06f1a09e8a8c983bf25d61c47c1f4e

Generation Details

Checking non nullable primitive for null.

existing (compile error)

public PolymorphicProperty(bool actualInstance)
{
	//...
	this.ActualInstance = actualInstance ?? throw new ArgumentException("Invalid instance found. Must not be null.");
}

for primitives should not have the conditional check, i.e.

public PolymorphicProperty(bool actualInstance)
{
	//...
	this.ActualInstance = actualInstance;
}

XML Comments should be escaped to accomodate to Generic <> Types

existing

/// <summary>
/// Initializes a new instance of the <see cref="PolymorphicProperty" /> class
/// with the <see cref="List<string>" /> class
/// </summary>
/// <param name="actualInstance">An instance of List<string>.</param>

better

/// <summary>
/// Initializes a new instance of the <see cref="PolymorphicProperty" /> class
/// with the <see cref="List&lt;string&gt;" /> class
/// </summary>
/// <param name="actualInstance">An instance of List&lt;string&gt;.</param>

Getter for Generic Type has an invalid method name

existing (won't compile)

public List<string> GetList<string>()

should maybe be:

public List<string> GetListString()

Steps to reproduce

Generate csharp code:

java -jar openapi-generator-cli.jar generate -i firefly-iii-1.5.5.yaml -g csharp-netcore --library httpclient

Related issues/PRs

Suggest a fix

Start by running the documented openapi-generator-cli command with the linked Firefly schema or gist and inspect the generated csharp-netcore output. Check the OneOf constructor, XML comments, and generic-type getter; done means the generated C# compiles, primitive values are not null-checked, XML generics are escaped, and getter names are valid.