[codex] Emit MCP tool calls as turn items (#20677)

## Why

`McpToolCall` was still an app-server item synthesized from deprecated
legacy begin/end events. Recent item migrations moved this ownership
into core `TurnItem`s, 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`

codex-rs/app-server-protocol/src/protocol/thread_history.rs

@@ -359,6 +359,7 @@ impl ThreadHistoryBuilder {
             | codex_protocol::items::TurnItem::ImageView(_)
             | codex_protocol::items::TurnItem::ImageGeneration(_)
             | codex_protocol::items::TurnItem::FileChange(_)
+            | codex_protocol::items::TurnItem::McpToolCall(_)
             | codex_protocol::items::TurnItem::ContextCompaction(_) => {}
         }
     }
@@ -382,6 +383,7 @@ impl ThreadHistoryBuilder {
             | codex_protocol::items::TurnItem::ImageView(_)
             | codex_protocol::items::TurnItem::ImageGeneration(_)
             | codex_protocol::items::TurnItem::FileChange(_)
+            | codex_protocol::items::TurnItem::McpToolCall(_)
             | codex_protocol::items::TurnItem::ContextCompaction(_) => {}
         }
     }