🧹 fix: Sanitize Artifact Filenames in Code Execution Output (#12222)

* fix: sanitize artifact filenames to prevent path traversal in code output

* test: Mock sanitizeFilename function in process.spec.js to return the original filename

- Added a mock implementation for the `sanitizeFilename` function in the `process.spec.js` test file to return the original filename, ensuring that tests can run without altering the filename during the testing process.

* fix: use path.relative for traversal check, sanitize all filenames, add security logging

- Replace startsWith with path.relative pattern in saveLocalBuffer, consistent
  with deleteLocalFile and getLocalFileStream in the same file
- Hoist sanitizeFilename call before the image/non-image branch so both code
  paths store the sanitized name in MongoDB
- Log a warning when sanitizeFilename mutates a filename (potential traversal)
- Log a specific warning when saveLocalBuffer throws a traversal error, so
  security events are distinguishable from generic network errors in the catch

* test: improve traversal test coverage and remove mock reimplementation

- Remove partial sanitizeFilename reimplementation from process-traversal tests;
  use controlled mock returns to verify processCodeOutput wiring instead
- Add test for image branch sanitization
- Use mkdtempSync for test isolation in crud-traversal to avoid parallel worker
  collisions
- Add prefix-collision bypass test case (../user10/evil vs user1 directory)

* fix: use path.relative in isValidPath to prevent prefix-collision bypass

Pre-existing startsWith check without path separator had the same class
of prefix-collision vulnerability fixed in saveLocalBuffer.

api/server/services/Files/Local/__tests__/crud-traversal.spec.js

@@ -0,0 +1,69 @@
+jest.mock('@librechat/api', () => ({ deleteRagFile: jest.fn() }));
+jest.mock('@librechat/data-schemas', () => ({
+  logger: { warn: jest.fn(), error: jest.fn() },
+}));
+
+const mockTmpBase = require('fs').mkdtempSync(
+  require('path').join(require('os').tmpdir(), 'crud-traversal-'),
+);
+
+jest.mock('~/config/paths', () => {
+  const path = require('path');
+  return {
+    publicPath: path.join(mockTmpBase, 'public'),
+    uploads: path.join(mockTmpBase, 'uploads'),
+  };
+});
+
+const fs = require('fs');
+const path = require('path');
+const { saveLocalBuffer } = require('../crud');
+
+describe('saveLocalBuffer path containment', () => {
+  beforeAll(() => {
+    fs.mkdirSync(path.join(mockTmpBase, 'public', 'images'), { recursive: true });
+    fs.mkdirSync(path.join(mockTmpBase, 'uploads'), { recursive: true });
+  });
+
+  afterAll(() => {
+    fs.rmSync(mockTmpBase, { recursive: true, force: true });
+  });
+
+  test('rejects filenames with path traversal sequences', async () => {
+    await expect(
+      saveLocalBuffer({
+        userId: 'user1',
+        buffer: Buffer.from('malicious'),
+        fileName: '../../../etc/passwd',
+        basePath: 'uploads',
+      }),
+    ).rejects.toThrow('Path traversal detected in filename');
+  });
+
+  test('rejects prefix-collision traversal (startsWith bypass)', async () => {
+    fs.mkdirSync(path.join(mockTmpBase, 'uploads', 'user10'), { recursive: true });
+    await expect(
+      saveLocalBuffer({
+        userId: 'user1',
+        buffer: Buffer.from('malicious'),
+        fileName: '../user10/evil',
+        basePath: 'uploads',
+      }),
+    ).rejects.toThrow('Path traversal detected in filename');
+  });
+
+  test('allows normal filenames', async () => {
+    const result = await saveLocalBuffer({
+      userId: 'user1',
+      buffer: Buffer.from('safe content'),
+      fileName: 'file-id__output.csv',
+      basePath: 'uploads',
+    });
+
+    expect(result).toBe('/uploads/user1/file-id__output.csv');
+
+    const filePath = path.join(mockTmpBase, 'uploads', 'user1', 'file-id__output.csv');
+    expect(fs.existsSync(filePath)).toBe(true);
+    fs.unlinkSync(filePath);
+  });
+});