[Bug]: Short model names (aliases) not supported on Responses API

[khoaatra]

Bug Description

Requests to /v1/responses fail with 404 model_not_found when using the short model name grok-4, but succeed when using the fully-qualified version grok-4-0709. If grok-4 is intended as an alias for the latest grok-4-* build, the alias is not being resolved. If aliases are not supported, the error message is misleading (it implies no worker is available for the model at all).

Related PR: #1117

Steps to Reproduce

Run SMG locally on port 9999, then call /v1/responses with "model": "grok-4":

curl -i http://localhost:9999/v1/responses \
  -H "Content-Type: application/json" \
  -H "opc-conversation-store-id: test-store-0326-01" \
  -H "Authorization: Bearer " \
  -d '{
    "model": "grok-4",
    "input": "Tell me about quang nam, vietnam"
  }'

Actual Behavior

HTTP/1.1 404 Not Found
content-type: application/json
x-smg-error-code: model_not_found
x-request-id: resp-1um1xAt9dH6i7K06DF1rDO0H

{"error":{"type":"Not Found","code":"model_not_found","message":"No worker available for model 'grok-4'","param":null}}

Expected Behavior

The request should resolve grok-4 which maps to latest grok model in the same family like it call xAI directly, and return 200 OK, similar to what happens when the full model name is sent explicitly:

curl -i http://localhost:9999/v1/responses \
  -H "Content-Type: application/json" \
  -H "opc-conversation-store-id: test-store-0326-01" \
  -H "Authorization: Bearer " \
  -d '{
    "model": "grok-4",
    "input": "Tell me about quang nam, vietnam"
  }'

HTTP/1.1 200 OK
content-type: application/json
x-request-id: resp-r0QcepBPFqZTRTS7fml8XePJ

{"created_at":1776116496,"completed_at":1776116510,"id":"2ad40d62-c655-d5c3-e03b-ffe19aa3fceb","model":"grok-4-0709","object":"response","output":[{"content":[{"type":"output_text","text":"### Overview of Quang Nam Province, Vietnam\n\nQuang Nam (often spelled Quảng Nam in Vietnamese) is a coastal province in central Vietnam..."}]}]}

Actual Behavior

HTTP/1.1 404 Not Found
content-type: application/json
x-smg-error-code: model_not_found
x-request-id: resp-1um1xAt9dH6i7K06DF1rDO0H

{"error":{"type":"Not Found","code":"model_not_found","message":"No worker available for model 'grok-4'","param":null}}

Component

model-gateway (core routing)

Routing Policy (if applicable)

None

Environment

• SMG: local build, port 9999
• Endpoint: /v1/responses

Deployment Environment

Local development

Streaming Context

• Issue occurs with streaming (SSE) responses
• Issue occurs with non-streaming responses
• Issue occurs in both modes

Additional Context

No response

Pre-submission Checklist

• I have searched existing issues and discussions
• I can reproduce this issue consistently
• I am using the latest version of SMG