Adds a defensive tool-pair-validator hook that runs as the final step in
the messages transform pipeline. When compaction or context-window recovery
removes user messages containing tool_result blocks without removing the
preceding assistant message with tool_use blocks, this validator detects
the mismatch and either:
1. Injects missing tool_result parts into the next user message, or
2. Creates a synthetic user message with placeholder tool_results
This prevents Anthropic API errors like 'tool_use ids found without
tool_result blocks immediately after'.
Fixes#3014
YeonGyu-Kim