fix: passthrough reasoning_content for DeepSeek thinking mode

[cnjack]

Problem

DeepSeek reasoning models (e.g. deepseek-reasoner, deepseek-v4-pro with thinking mode) return reasoning_content alongside content in API responses. Per DeepSeek API docs:

• When tool calls are involved, reasoning_content from assistant messages must be sent back in subsequent API requests
• Failure to do so results in a 400 Bad Request error

The toOpenAIMessage(), toEinoMessage(), and stream delta handling in chatmodel.go were all dropping the reasoning_content field, causing multi-turn tool-calling conversations with DeepSeek reasoning models to fail.

Fix

Map ReasoningContent in all three message conversion points:

1. toOpenAIMessage() — Eino Message → OpenAI request message (sends reasoning_content back to API)
2. toEinoMessage() — OpenAI response → Eino Message (captures reasoning_content from non-streaming responses)
3. Stream handler — stream delta → Eino Message (captures reasoning_content from streaming responses)

Both go-openai v1.41.2 and Eino v0.8.11 already support the ReasoningContent field — only the conversion layer was missing the mapping.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: bda5a3a7-1a96-4fdb-9b93-28bd2731c67e

📥 Commits

Reviewing files that changed from the base of the PR and between 9c5c912 and 6e84794.

📒 Files selected for processing (1)

• internal/model/chatmodel.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/deepseek-reasoning-content

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