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/server/src/core/client.ts

@@ -64,10 +64,13 @@ export class GeminiClient {
           .getTool('read_many_files') as ReadManyFilesTool;
         if (readManyFilesTool) {
           // Read all files in the target directory
-          const result = await readManyFilesTool.execute({
-            paths: ['**/*'], // Read everything recursively
-            useDefaultExcludes: true, // Use default excludes
-          });
+          const result = await readManyFilesTool.execute(
+            {
+              paths: ['**/*'], // Read everything recursively
+              useDefaultExcludes: true, // Use default excludes
+            },
+            AbortSignal.timeout(30000),
+          );
           if (result.llmContent) {
             initialParts.push({
               text: `\n--- Full File Context ---\n${result.llmContent}`,