fix(hook): add tool_use/tool_result pair validator to prevent API errors

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

src/plugin/messages-transform.ts

@@ -20,5 +20,9 @@ export function createMessagesTransformHandler(args: {
     await args.hooks.thinkingBlockValidator?.[
       "experimental.chat.messages.transform"
     ]?.(input, output)
+
+    await args.hooks.toolPairValidator?.[
+      "experimental.chat.messages.transform"
+    ]?.(input, output)
   }
 }