Support @dynamic=initialvalue for per-response retry-after values in GenericRandomErrorPlugin

[Copilot]

Extends the @dynamic token syntax for Retry-After headers to accept an optional initial value (@dynamic=N), enabling per-response retry-after configuration instead of relying solely on the global retryAfterInSeconds setting.

Changes

• Parse @dynamic=value syntax in UpdateProxyResponse, falling back to global config for plain @dynamic
• Made ShouldThrottle static with explicit retryAfterInSeconds parameter, captured via lambda for per-response values
• Added warning log for invalid @dynamic= values (non-integer)

Example

{
  "statusCode": 429,
  "headers": [
    {
      "name": "Retry-After",
      "value": "@dynamic=17"
    }
  ],
  "body": {
    "error": {
      "message": "Rate limit exceeded. Wait 17 seconds."
    }
  }
}

Uses 17 seconds for this response regardless of the plugin's global retryAfterInSeconds setting.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Feature: Support @dynamic=initialvalue for configurable initial retry-after value in GenericRandomErrorPlugin responses</issue_title>
<issue_description>## Proposal

Extend the GenericRandomErrorPlugin to enhance the handling of the retry-after response header. Currently, the initial value for retry-after is set globally for all responses in the plugin's configuration. This feature proposes supporting a syntax of @dynamic=initialvalue within the response header, enabling the initial retry-after value to be set on a per-response basis.

Motivation

• Allows greater flexibility and realism when simulating errors with varying backoff requirements.
• Developers can configure different endpoints to return different initial retry-after values, improving test coverage for clients handling these scenarios.

Acceptance Criteria

• The plugin should parse the @dynamic=initialvalue syntax from the retry-after response header configuration.
• If present, the initial retry-after value for that response should override the global default for that invocation.
• Documentation should be updated with usage examples.

Example

{
  "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v0.29.0/genericrandomerrorplugin.errorsfile.schema.json",
  "errors": [
    {
      "request": {
        "url": "https://models.github.ai/inference/chat/completions",
        "method": "POST"
      },
      "responses": [
        {          
          "statusCode": 429,
          "headers": [
            {
              "name": "Content-Type",
              "value": "application/json; charset=utf-8"
            },
            {
              "name": "Retry-After",
              "value": "@dynamic=17"
            }
          ],
          "body": {
            "error": {
              "code": "RateLimitReached",
              "message": "Rate limit of 15 per 60s exceeded for UserByModelByMinute. Please wait 17 seconds before retrying.",
              "details": "Rate limit of 15 per 60s exceeded for UserByModelByMinute. Please wait 17 seconds before retrying."
            }
          }
        }
      ]
    }
  ]
}

In this example, the initial retry-after value will be set to 17 for this response, regardless of the global plugin setting.
</issue_description>

Comments on the Issue (you are @copilot in this section)

@garrytrinder Good idea! 👍

• Fixes Feature: Support @dynamic=initialvalue for configurable initial retry-after value in GenericRandomErrorPlugin responses #1272

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[Copilot]

Pull request overview

This PR extends the GenericRandomErrorPlugin to support per-response retry-after values using the @dynamic=N syntax in response headers. This allows developers to configure different initial retry-after values for individual error responses instead of relying solely on the global retryAfterInSeconds configuration setting.

Changes:

• Enhanced header value parsing to support @dynamic=value syntax with fallback to global config for plain @dynamic
• Refactored ShouldThrottle method to be static with explicit retry-after parameter, captured via lambda for per-response values
• Added warning logging for invalid @dynamic= values