[core] Drop parent MCP lifecycle events from forked agent history

[chess-oai]

summary

• exclude inherited McpToolCallBegin and McpToolCallEnd events when constructing forked agent history
• preserve the complete MCP history in the parent rollout while keeping parent tool execution state out of the child
• drop both lifecycle halves so legacy histories cannot create an orphaned in-progress tool call

why

#16709 stripped parent tool and reasoning items from forked history, but retained every EventMsg

MCP results are also persisted in McpToolCallEnd events and may contain large text, structured output, or embedded resources. Forks that retain those parent events serialize the same result payloads into each child rollout, multiplying disk writes, rollout size, and later read and parse work as agent fan-out grows

These events describe calls already executed by the parent and do not contribute to the child model context, so filtering them completes the existing fork sanitizer boundary without changing active MCP execution or parent history

testing

• just test -p codex-core forked_rollout_item_filter_drops_mcp_tool_call_lifecycle
• just test -p codex-core spawn_agent_can_fork_parent_thread_history_with_sanitized_items
• just fmt

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