fix(deepseek): ensure reasoning_content is included in multi-turn tool chains

[dixoxib]

Background

When using DeepSeek's thinking mode with multi-turn tool calls, the reasoning_content field was not being properly included in assistant messages. DeepSeek API which expects reasoning_content to be present in all assistant messages within the current thinking chain, even if empty, else responding with error 400.

Summary

Modified packages/deepseek/src/chat/convert-to-deepseek-chat-messages.ts to ensure reasoning_content is included for every assistant message in the current thinking chain when thinking mode is enabled. The fix ensures that when thinking mode is active and tool calls are present, an empty reasoning_content string is provided if no reasoning content exists, maintaining API compatibility with DeepSeek's requirements.

Manual Verification

• Verified that existing test fixtures for reasoning mode continue to work correctly
• Tested multi-turn conversations with tool calls in thinking mode using the DeepSeek provider
• Confirmed that the fix doesn't break non-thinking mode conversations
• Validated that JSON response format handling remains unchanged

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Future Work

Fix edge cases on DeepSeek API behavior if there's a need.