feat: add reasoning middleware

[snipeship]

Closes #6337, #4033, #3703

---

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
    },
  }
}

---

Before (see issue)

After:

[snipeship]

as discussed here:
https://discord.com/channels/1391832426048651334/1394667004979445931/1455066042056441982
cc @rekram1-node

[rekram1-node]

wondering if there is anyway we could avoid additional configs and just always have it on, and then default to handling which should cover 90% of users and they wont have to do configs manually?