fix(tracing): capture completed McpToolCall items as named tool calls

[lakshya-dhariwal]

Fixes langfuse/langfuse#17969

What

Desktop code-mode turns emit nested MCP calls only as completed McpToolCall items on item_completed - there are no mcp_tool_call_begin/end events for them and no model-visible tool call to enrich. parse.ts only handled item_completed for UserMessage, so those calls never became observations: traces showed the outer TOOL exec but nothing for the MCP call inside it.

Change

Handle item_completed with item.type === "McpToolCall" in the parser:

• New tool call named server.tool (the existing mcp naming path in trace.ts renders it the same way) with arguments as input and result as output.
• status: "failed" records an error instead of an output, unwrapping Err/string error shapes.
• Deduplicates by call ID: if a model-visible call with the same ID exists (the ordinary CLI path), the item enriches it with MCP identity instead of creating a duplicate - same role as mcp_tool_call_begin/end.
• startTime uses the completion timestamp. The item carries a duration, but its serialized shape/units are not documented, so this does not guess from it; the span is zero-length rather than wrong.

Testing

• Two new parser tests with synthetic payloads, matching the shapes in the issue: a code-mode-style completed McpToolCall becomes one codex_app.list_projects tool call with input/output; a completed item whose ID matches a model-visible function_call enriches it (MCP identity + failure error) without duplicating it. Both fail without the parser change.
• pnpm test - 122/122 pass. pnpm run lint (format + tsc + build) clean.

Notes

• Reproduced the parsing gap against the event shapes quoted in bug: Codex Desktop code-mode MCP calls omitted from tracing plugin langfuse#17969; I don't have a Codex Desktop setup here, so end-to-end verification against a live Desktop turn would be a useful maintainer check.
• Implemented with AI assistance (Claude) acting on the contributor's behalf; contributor directed and reviewed the change.

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[lakshya-dhariwal]

Friendly bump. This captures completed McpToolCall items as native tool spans, and the CLA is signed. Could a maintainer take a look when you have time? Happy to adjust.