[BUG] [C#] model-name-prefix combined with polymorhphism & discriminator generates code that does not compile

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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?

Description

openapi-generator version

7.15.0

OpenAPI declaration file content or url

I have isolated just the one endpoint w/ request parameters that appears to reproduce this bug. It was encountered in the Ory project (https://github.com/ory/sdk/issues/434).

mre.json

Generation Details

$ OPENAPI_GENERATOR_CLI_SEARCH_URL=DEFAULT npx @openapitools/openapi-generator-cli@2.23.1 version-manager set 7.15.0
$ npx @openapitools/openapi-generator-cli@2.23.1 generate -i mre.json -g csharp --model-name-prefix="Client"

Steps to reproduce

Run CLI commands above, pointing at mre.json

Open ClientUpdateRecoveryFlowBody.json

Observe invalid C# in the Read override method for the System.Text.Json.JsonConverter:

public override ClientUpdateRecoveryFlowBody Read(ref Utf8JsonReader utf8JsonReader, Type typeToConvert, JsonSerializerOptions jsonSerializerOptions)
{
    int currentDepth = utf8JsonReader.CurrentDepth;
 
    if (utf8JsonReader.TokenType != JsonTokenType.StartObject && utf8JsonReader.TokenType != JsonTokenType.StartArray)
        throw new JsonException();
 
    JsonTokenType startingTokenType = utf8JsonReader.TokenType;
 
    Option<ClientUpdateRecoveryFlowBody.MethodEnum?> method = default;
 
    Utf8JsonReader utf8JsonReaderDiscriminator = utf8JsonReader;
    while (utf8JsonReaderDiscriminator.Read())
    {
        if (startingTokenType == JsonTokenType.StartObject && utf8JsonReaderDiscriminator.TokenType == JsonTokenType.EndObject && currentDepth == utf8JsonReaderDiscriminator.CurrentDepth)
            break;
 
        if (startingTokenType == JsonTokenType.StartArray && utf8JsonReaderDiscriminator.TokenType == JsonTokenType.EndArray && currentDepth == utf8JsonReaderDiscriminator.CurrentDepth)
            break;
 
        if (utf8JsonReaderDiscriminator.TokenType == JsonTokenType.PropertyName && currentDepth == utf8JsonReaderDiscriminator.CurrentDepth - 1)
        {
            string? localVarJsonPropertyName = utf8JsonReaderDiscriminator.GetString();
            utf8JsonReaderDiscriminator.Read();
            if (localVarJsonPropertyName?.Equals("method") ?? false)
            {
                string? discriminator = utf8JsonReaderDiscriminator.GetString();
                if (discriminator?.Equals("code") ?? false)
                {
                    Utf8JsonReader utf8JsonReader = utf8JsonReader;
                     = JsonSerializer.Deserialize<>(ref utf8JsonReader, jsonSerializerOptions); // fails to compile
                }
                if (discriminator?.Equals("link") ?? false)
                {
                    Utf8JsonReader utf8JsonReader = utf8JsonReader;
                     = JsonSerializer.Deserialize<>(ref utf8JsonReader, jsonSerializerOptions); // fails to compile
                }
            }
        }
    }
    // ...

If --model-name-prefix not specified, it works correctly.

Start by running the documented openapi-generator CLI command with mre.json and --model-name-prefix="Client". Inspect the generated ClientUpdateRecoveryFlowBody.json and the C# generator path handling polymorphism, discriminators, and model-name prefixes; done means the generated C# compiles, while generation without the prefix continues to work.