Add structured outputs support to Runpod API

[endolith]

Add support for structured outputs (JSON schema, regex, choice, grammar,
and structural_tag constraints) using vLLM 0.11.0+'s
StructuredOutputsParams API.

Changes:

• Import StructuredOutputsParams from vllm.sampling_params
• Extract structured_outputs from extra_body in sampling_params
• Create StructuredOutputsParams instances for json, regex, choice,
  grammar, and structural_tag constraint types
• Reject deprecated guided_json parameter (from vLLM <0.11.0) with
  clear error message pointing to migration guide

This enables users to enforce JSON schemas and other structured output
formats on model responses, as documented in vLLM 0.11.0.

https://docs.vllm.ai/en/v0.11.0/features/structured_outputs.html

Fixes #235

[endolith]

This was written by AI but seems to work, with the possible exception of the structural_tag. I need to check more carefully. It also doesn't enforce field order in json, which might be a vLLM issue.

[endolith]

with the possible exception of the structural_tag.

I think this was just a misunderstanding of the AI verifying the outputs? structural_tag only constrains text within the start and end tags, but allows arbitrary text outside of it?

I tested it with a bunch of contradictory prompts to prove that the output was actually being constrained to the schema:

• Test script
• Test report

There are some discrepancies, like:

• In the "JSON schema with optional fields" example, the LLM doesn't seem to ever use the optional fields, even when specifically told to.
• Also the "JSON object" mode seems to allow extraneous text outside the JSON object, which I'm not sure is as intended.

But those are probably all vLLM or Outlines/LMFE issues, not worker-vllm issues?

There may also be an issue with newlines not being escaped correctly in json output? I'm not sure if that's the fault of my script, or worker-vllm, or vLLM itself, or Outlines/LMFE.

If I go on the Runpod Requests tab and request

{
  "input": {
    "messages": [
      {
        "role": "system",
        "content": "IMPORTANT: When outputting JSON, use actual newlines within string values, not escaped \\n characters. For example, use:\n{\n  \"text\": \"This is line one\nThis is line two\nThis is line three\"\n}\nNever use \\n escapes in your JSON string values."
      },
      {
        "role": "user",
        "content": "Output a JSON object with a multi-line text field describing the Eiffel Tower"
      }
    ],
    "sampling_params": {
      "max_tokens": 200,
      "temperature": 0.1,
      "extra_body": {
        "structured_outputs": {
          "json": {
            "type": "object",
            "properties": {
              "description": {
                "type": "string",
                "description": "Multi-line description"
              },
              "year_built": {
                "type": "integer"
              }
            },
            "required": [
              "description",
              "year_built"
            ]
          }
        }
      }
    }
  }
}

it ignores the prompt and still escapes newlines:

{
  "delayTime": 61,
  "executionTime": 4479,
  "id": "7d5bacca-3d9f-4bc8-84bf-f1d44d078bd8-u2",
  "output": [
    {
      "choices": [
        {
          "tokens": [
            "{\n  \"description\": \"The Eiffel Tower is a wrought-iron lattice tower on the Champ de Mars in Paris, France. It is named after the engineer Gustave Eiffel, whose company designed and built the tower.\\n\\nStanding at 330 meters tall, it was the tallest man-made structure in the world until the completion of the Chrysler Building in New York in 1930. The Eiffel Tower is now a global cultural icon of France and one of the most recognizable structures in the world.\\n\\nThe tower has three levels for visitors, with restaurants on the first and second levels. The top level's upper platform is 276 meters above the ground, accessible by staircase or elevator. The tower's design includes a series of open-lattice platforms, which allow wind to pass through and reduce the swaying of the structure.\\n\\"
          ]
        }
      ],
      "usage": {
        "input": 90,
        "output": 200
      }
    }
  ],
  "status": "COMPLETED",
  "workerId": "1ut1hgflhf242q"
}

so I'm not sure why I'm seeing unescaped newlines occasionally. I made tests for them and they all passed.

[endolith]

Actually, it seems Structured Outputs are already supported and working (somewhat...) when using the OpenAI API, and I confused that with the RunPod Job API.

I thought this was fixing something broken, but it's actually introducing something new, and tacking on OpenAI API concepts to a different API.

AI says:

What I proposed:
I added support for structured outputs to the RunPod Job API by accepting extra_body in sampling_params and converting it to StructuredOutputsParams. This is new functionality, not a bug fix.

Alternative approach (Option 2):
If structured outputs should be added to the RunPod Job API, a more appropriate format would match vLLM's native offline inference format:

"sampling_params": {
    "max_tokens": 500,
    "structured_outputs": {
        "json": {...}
    }
}

This would convert {"json": {...}} → StructuredOutputsParams(json={...}) directly, without borrowing the OpenAI-specific extra_body concept.

[Actually vLLM's implementation seems to be broken and doesn't always constrain outputs perfectly, but that's another issue.]

[endolith]

Changed it to map the offline inference interface directly instead of mixing OpenAI API into Runpod API

(and rebased onto #244)