[mcp] Support MCP Apps part 2 - Add meta to mcp tool call result. (#16465)

- [x] Add meta to mcp tool call result.

codex-rs/exec/tests/event_processor_with_json_output.rs

@@ -482,6 +482,7 @@ fn mcp_tool_call_begin_and_end_emit_item_events() {
                 result: Some(McpToolCallResult {
                     content: Vec::new(),
                     structured_content: None,
+                    meta: None,
                 }),
                 error: None,
                 duration_ms: Some(1_000),
@@ -613,6 +614,7 @@ fn mcp_tool_call_defaults_arguments_and_preserves_structured_content() {
                         "text": "done",
                     })],
                     structured_content: Some(json!({ "status": "ok" })),
+                    meta: None,
                 }),
                 error: None,
                 duration_ms: Some(10),