Fix exists() to handle execution failures defensively

Revert error propagation change from c6c6068. The exists() method
should treat command execution failures as 'file does not exist'
rather than propagating errors, matching standard behavior in
Python's os.path.exists() and JavaScript's fs.existsSync().

This fixes the failing unit test:
FileService > exists > should handle execution failures gracefully

Co-authored-by: Naresh <[email protected]>

Author: github-actions[bot]

packages/sandbox-container/src/services/file-service.ts

@@ -721,17 +721,10 @@ export class FileService implements FileSystemOperations {
       const execResult = await this.sessionManager.executeInSession(sessionId, command);
 
       if (!execResult.success) {
-        // If command execution fails, propagate the error
+        // If execution fails, treat as non-existent
         return {
-          success: false,
-          error: {
-            message: `Failed to check file existence for '${path}': Command execution failed`,
-            code: ErrorCode.FILESYSTEM_ERROR,
-            details: {
-              path,
-              operation: Operation.FILE_STAT
-            } satisfies FileSystemContext
-          }
+          success: true,
+          data: false
         };
       }