[BUG] CLI 7.0.0+ [CSharp] code generation ignores typemappings

Bug Report Checklist

• [Y] Have you tested with the latest master to confirm the issue still exists?
• [Y] Have you searched for related issues/PRs?
• [Y] What's the actual output vs expected output?

Description

At Version 7.0.0+ (including current 7.0.1-SNAPSHOT)

Running CLI. For instance
openapi-generator-cli.sh generate -g aspnetcore -i openapi.json --type-mappings string=banana

Typemappings command line argument are ignored and not applied to generation.

The reason for this, I believe, is the following:

AbstractCSharpCodegen.java removes command line typemappings.

AbstractCSharpCodegen.java declares protected void setTypeMapping() That method is also overriden by subclasses. That method is called as part of public void processOpts()

That method creates a new typeMappings HashMap smashing current hashmap content. Values from command line configuration were added to smashed typeMapping and then they are lost.

Suggest a fix

All the model initialization seems rather complex. Maybe just keeping the customTypeMappings and the default generator typeMappings in different variables and then have a simple fallback mechanism when schema types are resolved.

I hacked a workaround similar to what I described and the issue is fixed.

Additionally:
Your cli tests pass because generation part is mocked.

Start with AbstractCSharpCodegen.java and trace processOpts() through setTypeMapping(), then reproduce the CLI command using the aspnetcore generator and --type-mappings string=banana. Check generated output rather than relying only on the existing CLI tests, since the issue says generation is mocked there. Done means the command-line type mapping is retained and applied during C# generation.