[BUG][csharp][restsharp] throwOnAnyError: oneOf/anyOf deserialization failure on 2xx escapes as InvalidDataException instead of ApiException #25218

Description

With throwOnAnyError=true the generated ApiClient promises that every failure surfaces as ApiException. For a 2xx response whose body matches none of the oneOf/anyOf variants, FromJson throws InvalidDataException after the ErrorException guard has already passed, so the raw exception reaches the caller without status code, body or headers.

openapi-generator version

master (after #25217)

Steps to reproduce

1. Generate a csharp client with --library restsharp --additional-properties throwOnAnyError=true from a spec where an operation returns a oneOf model.
2. Call that operation against a server that returns 200 with a body matching no variant.
3. Observe InvalidDataException instead of ApiException.

Expected behaviour

ApiException with ErrorCode = 200, ErrorContent = raw body, Headers populated, and the InvalidDataException as InnerException.

Original findings

P2: The FromJson call below can throw InvalidDataException for a 2xx body that matches no oneOf/anyOf schema, after this check has passed; the raw exception escapes instead of the promised ApiException. Wrap that parse failure with the response status, content, and headers.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. When an issue isn't valid or won't be fixed in this PR, reply in its thread with the reason and then resolve the thread. At modules/openapi-generator/src/main/resources/csharp/ApiClient.mustache, line 500:

<comment>The `FromJson` call below can throw `InvalidDataException` for a 2xx body that matches no oneOf/anyOf schema, after this check has passed; the raw exception escapes instead of the promised `ApiException`. Wrap that parse failure with the response status, content, and headers.</comment>

<file context>
@@ -495,6 +494,14 @@ namespace {{packageName}}.Client
+                {{#throwOnAnyError}}
+
+                // 4xx/5xx also carry ErrorException; those go through ExceptionFactory so the body is kept
+                if (response.ErrorException != null && (int)response.StatusCode < 400)
+                {
+                    throw new ApiException((int)response.StatusCode, string.Format("Error calling {0}: {1}", request.Resource, response.ErrorMessage), response.Content, ToApiResponse(response).Headers);
</file context>

Originally posted by @cubic-dev-ai[bot] in #25217