feat: add reasoning middleware

[snipeship]

Closes #6337, #4033, #3703

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Adds a new interleaved object shape to model config with tagName (required) and optional startWithReasoning. Read ai-sdk's extractReasoningMiddleware() as a reference.

Example config:

"models": {
  "model-name": {
    "reasoning": true,
    "interleaved": {
      "tagName": "think" // or other XML tag
      // "startWithReasoning": true
    },
  }
}

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Before (see issue)

After: