OpenAPIGenerator upgrade to v7.16.0 - [2] Added new mustache templates

[Kwok-he-Chu]

PR-2 of #1204

Description

This PR introduces the mustache.templates needed for the upgrade of the OpenApiGenerator to v7.16.0 used in the Adyen sdk-automation to generate .NET models & services from the Adyen OpenAPI Specifications.

We've always removed the .NET6.0 (= end of life support) workflow as we'll continue on .NET 8.0+

[gemini-code-assist]

Code Review

This pull request is a significant upgrade, replacing old templates with new ones for the generichost library to support OpenAPI Generator v7.16.0. The changes are extensive and introduce a new structure for C# code generation. My review of the new mustache templates has identified a few critical and high-severity issues related to null handling, incorrect logic in authentication flows, and attempts to instantiate abstract classes. These could lead to runtime exceptions or unexpected behavior in the generated code. I've provided detailed feedback and code suggestions to improve the robustness and correctness of the new templates.

[gemini-code-assist]

This catch block has several potential NullReferenceException issues and lacks proper error handling for encrypted keys without a passphrase.

1. str.ReadLine() can return null, but the null-forgiving operator (!) is used, which can lead to a crash. You should use null-conditional access (?.).
2. If an encrypted key is provided without a keyPassPhrase, the code will crash at line 450. There should be a check for a null keyPassPhrase before attempting to decrypt the key.

I suggest refactoring this block for safer null handling and clearer error reporting.

            catch (global::System.FormatException)
            {
                StringReader str = new StringReader(pvkstr);

                //-------- read PEM encryption info. lines and extract salt -----
                if (str.ReadLine()?.StartsWith("Proc-Type: 4,ENCRYPTED") != true)
                    return null;
                
                String saltline = str.ReadLine();
                if (saltline?.StartsWith("DEK-Info: DES-EDE3-CBC,") != true)
                    return null;
                
                String saltstr = saltline.Substring(saltline.IndexOf(",") + 1).Trim();
                byte[] salt = new byte[saltstr.Length / 2];
                for (int i = 0; i < salt.Length; i++)
                    salt[i] = Convert.ToByte(saltstr.Substring(i * 2, 2), 16);

                if (!(str.ReadLine() == ""))
                    return null;

                //------ remaining b64 data is encrypted RSA key ----
                String encryptedstr = str.ReadToEnd();

                try
                {   //should have b64 encrypted RSA key now
                    binkey = Convert.FromBase64String(encryptedstr);
                }
                catch (global::System.FormatException)
                {   //data is not in base64 format
                    return null;
                }

                if (keyPassPhrase == null)
                    throw new ArgumentException("A passphrase is required for an encrypted private key.", nameof(keyPassPhrase));

                byte[] deskey = GetEncryptedKey(salt, keyPassPhrase, 1, 2);    // count=1 (for OpenSSL implementation); 2 iterations to get at least 24 bytes
                if (deskey == null)
                    return null;

                //------ Decrypt the encrypted 3des-encrypted RSA private key ------
                byte[]{{nrt?}} rsakey = DecryptKey(binkey, deskey, salt); //OpenSSL uses salt value in PEM header also as 3DES IV

                return rsakey;
            }

[Kwok-he-Chu]

This template is used in two test files, I've removed them here

[gemini-code-assist]

The current implementation of IsValidHmacSignature has a critical flaw: it uses the _adyenHmacKey field which is never initialized, causing HMAC validation to always use a null key.

To fix this and follow best practices, I recommend the following changes:

1. Store the provider: In the {{apiName}}Handler constructor, store the injected ITokenProvider<HmacKeyToken> in a private readonly field (e.g., _hmacKeyProvider).
2. Make validation async: Change IsValidHmacSignature to be an async method (e.g., IsValidHmacSignatureAsync) in both the {{interfacePrefix}}{{apiName}}Handler interface and the {{apiName}}Handler class. This is crucial to avoid blocking on I/O when retrieving the key.
3. Implement async logic: In the new async method, await the key from _hmacKeyProvider.GetAsync() and then perform the validation.

This approach ensures correct, non-blocking HMAC validation.

[gemini-code-assist]

The default case in this switch statement does nothing, which means if an unsupported OAuthFlow is provided, the _grantType field will remain uninitialized. This will likely cause a null value to be sent in the token request later. It's better to throw an exception for unsupported flows to fail early and clearly.

                default:
                    throw new NotSupportedException($"The OAuth flow '{flow}' is not supported.");

[Kwok-he-Chu]

This mustache template is unused

[Kwok-he-Chu]

@gcatanese Q: Do we leave this in? I've only included it for completeness as part of the v7 generator upgrade

[Kwok-he-Chu]

@gcatanese The mustache templates include the generated templates for the classes in the /Core-folder, these templates can be removed (e.g. Options.mustache template etc.)