Enable tools to cancel active execution.

- Plumbed abort signals through to tools
- Updated the shell tool to properly cancel active requests by killing the entire child process tree of the underlying shell process and then report that the shell itself was canceled.

Fixes https://b.corp.google.com/issues/416829935

packages/cli/src/ui/hooks/atCommandProcessor.ts

@@ -26,6 +26,7 @@ interface HandleAtCommandParams {
   addItem: UseHistoryManagerReturn['addItem'];
   setDebugMessage: React.Dispatch<React.SetStateAction<string>>;
   messageId: number;
+  signal: AbortSignal;
 }
 
 interface HandleAtCommandResult {
@@ -90,6 +91,7 @@ export async function handleAtCommand({
   addItem,
   setDebugMessage,
   messageId: userMessageTimestamp,
+  signal,
 }: HandleAtCommandParams): Promise<HandleAtCommandResult> {
   const trimmedQuery = query.trim();
   const parsedCommand = parseAtCommand(trimmedQuery);
@@ -163,7 +165,7 @@ export async function handleAtCommand({
   let toolCallDisplay: IndividualToolCallDisplay;
 
   try {
-    const result = await readManyFilesTool.execute(toolArgs);
+    const result = await readManyFilesTool.execute(toolArgs, signal);
     const fileContent = result.llmContent || '';
 
     toolCallDisplay = {