fix: track reasoning by output_index for copilot compatibility

[NateSmyth]

Problem

Copilot changes the item_id on every event, meaning reasoning delta get dropped because they don't match the stored ID.

Fix

Reuse the existing currentTextId pattern that is already used for text streaming

Testing

Verified with:

  "provider": {
    "github-copilot": {
      "models": {
        "gpt-5.2-codex": {
          "variants": {
            "reasoning-test": {
              "reasoningEffort": "high",
              "reasoningSummary": "auto",
              "include": [
                "reasoning.encrypted_content"
              ],
              "store": false
            }
          }
        }
      }
    }
  }

Throws an error pre-fix.

"reasoning part <long base64 string>:0 not found"

Post-fix shows reasoning blocks in the UI.

Screenshot

Only works for Codex models via the /responses endpoint.