Op agent: preserve tool-call protocol

Fix tool-call protocol preservation in Op agent loop.

Problem

Op agent currently collapses tool calls/results into text:

• Tool results are appended as PromptMessage "tool" (tshow result)
• Provider messages always set msgToolCalls/msgToolCallId to Nothing

This loses structured tool call semantics and can break providers that require tool_call_id or structured tool response messages.

Requirements

• Preserve tool_call_id, tool_calls structure across Op -> Provider -> Op
• Ensure tool response messages include correct tool_call_id and content
• Keep backward compatibility with current tools

Implementation Ideas

• Extend Op.PromptMessage to carry optional tool metadata
• Map tool calls into Provider.msgToolCalls
• Map tool results into Provider.msgToolCallId + content

Acceptance

• Tool-using prompts work with providers that require tool_call_id
• Trace shows tool calls/results with proper ids