Use message string in v2 send_message

[jif-oai]

Summary

• switch MultiAgentV2 send_message to accept a single message string instead of items
• keep the old assign_task item parser in place for the next branch
• update send_message schema/spec and focused handler tests

Verification

• cargo test -p codex-tools send_message_tool_requires_message_and_uses_submission_output
• cargo test -p codex-core multi_agent_v2_send_message
• just fix -p codex-tools
• just fix -p codex-core
• just argument-comment-lint