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/hooks/create-transform-hooks.ts

@@ -5,6 +5,7 @@ import {
   createClaudeCodeHooksHook,
   createKeywordDetectorHook,
   createThinkingBlockValidatorHook,
+  createToolPairValidatorHook,
 } from "../../hooks"
 import {
   contextCollector,
@@ -17,6 +18,7 @@ export type TransformHooks = {
   keywordDetector: ReturnType<typeof createKeywordDetectorHook> | null
   contextInjectorMessagesTransform: ReturnType<typeof createContextInjectorMessagesTransformHook>
   thinkingBlockValidator: ReturnType<typeof createThinkingBlockValidatorHook> | null
+  toolPairValidator: ReturnType<typeof createToolPairValidatorHook> | null
 }
 
 export function createTransformHooks(args: {
@@ -63,10 +65,19 @@ export function createTransformHooks(args: {
       )
     : null
 
+  const toolPairValidator = isHookEnabled("tool-pair-validator")
+    ? safeCreateHook(
+        "tool-pair-validator",
+        () => createToolPairValidatorHook(),
+        { enabled: safeHookEnabled },
+      )
+    : null
+
   return {
     claudeCodeHooks,
     keywordDetector,
     contextInjectorMessagesTransform,
     thinkingBlockValidator,
+    toolPairValidator,
   }
 }

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)
   }
 }