Record MCP result telemetry on mcp.tools.call spans

[mchen-oai]

Why

• Without change: MCP tool call spans include request-side details such as server, tool, call ID, connector, session, and turn.
• Issue: Some useful telemetry is only known by the MCP server after it handles the tool call, such as target identity or whether the call triggered a user-facing flow.

What Changed

• With change: Codex reads allowlisted telemetry from _meta["codex/telemetry"]["span"] and records it on the mcp.tools.call span.
• Adds span fields for codex.mcp.target.id and codex.mcp.user_flow.triggered, with strict type checks and bounded target ID length.

Verification

codex-rs/core/src/mcp_tool_call_tests.rs