[codex] Emit MCP tool calls as turn items

[pakrym-oai]

Why

McpToolCall was still an app-server item synthesized from deprecated legacy begin/end events. Recent item migrations moved this ownership into core TurnItems, so MCP tool calls now follow the same canonical lifecycle and leave legacy events as compatibility fanout.

Keeping the core item close to the v2 ThreadItem::McpToolCall shape also avoids spreading MCP result semantics across app-server conversion code. Core now owns whether a completed call is completed or failed, and whether the payload is a tool result or an error.

What changed

• Added core TurnItem::McpToolCall with flattened server, tool, arguments, status, result, and error fields.
• Updated MCP tool call emitters, including MCP resource tools, to emit ItemStarted/ItemCompleted around directly constructed core MCP items.
• Updated app-server v2 conversion to project the core MCP item into ThreadItem::McpToolCall without deriving status or splitting Result locally.
• Ignored live deprecated MCP legacy fanout in app-server v2 to avoid duplicate item notifications, while keeping thread history replay on the legacy event path.

Verification

• cargo test -p codex-protocol
• cargo test -p codex-app-server-protocol
• cargo test -p codex-core --lib mcp_tool_call
• cargo check -p codex-app-server
• cargo test -p codex-app-server mcp_tool_call_completion_notification_contains_truncated_large_result