📏 refactor: Add File Size Limits to Conversation Imports (#12221)

* fix: add file size limits to conversation import multer instance

* fix: address review findings for conversation import file size limits

* fix: use local jest.mock for data-schemas instead of global moduleNameMapper

The global @librechat/data-schemas mock in jest.config.js only provided
logger, breaking all tests that depend on createModels from the same
package. Replace with a virtual jest.mock scoped to the import spec file.

* fix: move import to top of file, pre-compute upload middleware, assert logger.warn in tests

* refactor: move resolveImportMaxFileSize to packages/api

New backend logic belongs in packages/api as TypeScript. Delete the
api/server/utils/import/limits.js wrapper and import directly from
@librechat/api in convos.js and importConversations.js. Resolver unit
tests move to packages/api; the api/ spec retains only multer behavior
tests.

* chore: rename importLimits to import

* fix: stale type reference and mock isolation in import tests

Update typeof import path from '../importLimits' to '../import' after
the rename. Clear mockLogger.warn in beforeEach to prevent cross-test
accumulation.

* fix: add resolveImportMaxFileSize to @librechat/api mock in convos.spec.js

* fix: resolve jest.mock hoisting issue in import tests

jest.mock factories are hoisted above const declarations, so the
mockLogger reference was undefined at factory evaluation time. Use a
direct import of the mocked logger module instead.

* fix: remove virtual flag from data-schemas mock for CI compatibility

virtual: true prevents the mock from intercepting the real module in
CI where @librechat/data-schemas is built, causing import.ts to use
the real logger while the test asserts against the mock.

api/server/routes/__test-utils__/convos-route-mocks.js

@@ -3,6 +3,7 @@ module.exports = {
 
   api: (overrides = {}) => ({
     isEnabled: jest.fn(),
+    resolveImportMaxFileSize: jest.fn(() => 262144000),
     createAxiosInstance: jest.fn(() => ({
       get: jest.fn(),
       post: jest.fn(),