Checklist

• I searched related issues but found no solution.
• The bug persists in the latest version.
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Describe the bug

When dynamically registering external API workers (OpenAI) via POST /workers in IGW mode (--enable-igw), requests fail with 400 errors due to SGLang-specific parameters not being stripped.

Reproduction

# Start in IGW mode
cargo run --bin sgl-model-gateway -- --enable-igw --port 9999

# Register external worker
curl -X POST http://localhost:9999/workers \
  -d '{"url": "https://api.openai.com", "api_key": "sk-xxx", "runtime": "external"}'

# Request fails with 400
curl http://localhost:9999/v1/responses \
  -d '{"model": "gpt-4.1", "input": "Hello"}'

{
  "error": {
    "message": "Unknown parameter: 'priority'.",
    "type": "invalid_request_error",
    "param": "priority",
    "code": "unknown_parameter"
  }
}%

Environment

N/A - use command cargo run --bin sgl-model-gateway -- --enable-igw --port 9999 to start