fix(providers): serialize tool calls and tool results in conversation history

[Aaronontheweb]

Summary

• OpenAiCompatibleChatClient.ToMessage() was serializing all non-system messages as simple {"role": "...", "content": "..."} objects, dropping tool_calls from assistant messages and tool_call_id from tool result messages
• This broke multi-turn tool calling: the first tool call would succeed, but follow-up LLM requests contained malformed history causing the model to fall back to emitting tool calls as raw text
• Fix adds proper serialization for assistant messages with FunctionCallContent (emits tool_calls array) and tool result messages with FunctionResultContent (emits tool_call_id)

Test plan

• New test SerializesAssistantToolCalls_AndToolResults_InConversationHistory verifies correct payload structure
• All 6 existing OpenAiCompatibleChatClientTests pass
• Hit-tested against live lemonade/Qwen3.5 endpoint on big-gpu — single and multi-tool calls confirmed working via netclaw -p