[codex] Reject full-history spawn overrides

[md-oai]

Summary

This changes the MultiAgentV2 spawn_agent path to fail fast when a full-history fork (fork_turns: "all" or an omitted fork_turns) is combined with agent_type, model, or reasoning_effort overrides.

Full-history forks currently inherit the parent thread configuration. That means those child override fields are not applied in that path, which can make a session look like it requested an oracle/model override while the spawned child actually runs with the parent model. The guard turns that silent mismatch into a model-facing error and tells the caller to use fork_turns: "none" or a positive turn count when child role/model overrides need to apply.

The PR also updates the tool schema and root-agent prompt so agents get the rule before they call the tool, and it adjusts the tests that used to assert the old silent-ignore behavior.

Why this is useful

• Prevents misleading rollout traces where a tool call appears to request one agent/model but the spawned thread records another.
• Makes oracle/model override failures explicit and actionable instead of silently inheriting parent config.
• Preserves full-history fork behavior for plain context cloning while steering configurable subagents to fork modes that actually apply child config.

Validation

• cargo test -p codex-core multi_agent_v2_spawn -- --nocapture
• cargo test -p codex-tools spawn_agent_tool_v2_requires_task_name_and_lists_visible_models -- --nocapture
• cargo test -p codex-core root_agent_prompt_requires_explicit_fork_turns -- --nocapture
• git diff --check