feat: add reasoning and reasoning_content fields to OpenAI message types

Add support for reasoning fields in OpenAI-compatible chat completion
messages to enable compatibility with vLLM reasoning parsers.

Changes:
- Add `reasoning_content` and `reasoning` fields to OpenAIAssistantMessageParam
- Add `reasoning` field to OpenAIChoiceDelta (reasoning_content already existed)

Both field names are supported for maximum compatibility:
- `reasoning_content`: Used by vLLM ≤ v0.8.4
- `reasoning`: New field name in vLLM ≥ v0.9.x

vLLM documentation recommends migrating to the shorter `reasoning` field
name, but maintains backward compatibility with `reasoning_content`.

These fields allow reasoning models to return their chain-of-thought
process alongside the final answer, which is crucial for transparency
and debugging with reasoning models.

References:
- vLLM Reasoning Outputs: https://docs.vllm.ai/en/stable/features/reasoning_outputs/
- vLLM Issue #12468: https://github.com/vllm-project/vllm/issues/12468

Signed-off-by: Charlie Doern <cdoern@redhat.com>

docs/static/experimental-llama-stack-spec.yaml

@@ -859,6 +859,16 @@ components:
             type: array
           - type: 'null'
           nullable: true
+        reasoning_content:
+          anyOf:
+          - type: string
+          - type: 'null'
+          nullable: true
+        reasoning:
+          anyOf:
+          - type: string
+          - type: 'null'
+          nullable: true
       title: OpenAIAssistantMessageParam
       type: object
     OpenAIChatCompletionContentPartImageParam:
@@ -1588,6 +1598,11 @@ components:
           - type: string
           - type: 'null'
           nullable: true
+        reasoning:
+          anyOf:
+          - type: string
+          - type: 'null'
+          nullable: true
       title: OpenAIChoiceDelta
       type: object
     OpenAIChunkChoice:
@@ -7582,6 +7597,14 @@ components:
               $ref: '#/components/schemas/OpenAIChatCompletionToolCall'
             type: array
           - type: 'null'
+        reasoning_content:
+          anyOf:
+          - type: string
+          - type: 'null'
+        reasoning:
+          anyOf:
+          - type: string
+          - type: 'null'
       type: object
       title: OpenAIAssistantMessageParam
       description: A message containing the model's (assistant) response in an OpenAI-compatible chat completion request.
@@ -7611,6 +7634,14 @@ components:
               $ref: '#/components/schemas/OpenAIChatCompletionToolCall'
             type: array
           - type: 'null'
+        reasoning_content:
+          anyOf:
+          - type: string
+          - type: 'null'
+        reasoning:
+          anyOf:
+          - type: string
+          - type: 'null'
       type: object
       title: OpenAIAssistantMessageParam
       description: A message containing the model's (assistant) response in an OpenAI-compatible chat completion request.