## Summary
- attach the authoritative Codex thread id to MCP tool request
`_meta.threadId` for model-initiated tool calls
- attach the same thread id for manual `mcpServer/tool/call` requests
before invoking the MCP server
- cover both metadata helper behavior and the manual app-server MCP path
in tests
needed because the Rust app-server is the last place that still has
authoritative knowledge of “this model-generated MCP tool call belongs
to conversation/thread X” before the request leaves Codex and reaches
Hoopa. It adds threadId to MCP request metadata in the model-generated
tool-call path, using sess.conversation_id, and also does the same for
the manual mcpServer/tool/call path.
## Test plan
- `cargo test -p codex-core
mcp_tool_call_thread_id_meta_is_added_to_request_meta --lib`
- `cargo test -p codex-app-server
mcp_server_tool_call_returns_tool_result`
Paired Hoopa consumer PR: https://github.com/openai/openai/pull/833263
