👤 feat: User Placeholder Variables for Custom Endpoint Headers (#7993)

* 🔧 refactor: move `processMCPEnv` from `librechat-data-provider` and move to `@librechat/api`

* 🔧 refactor: Update resolveHeaders import paths

* 🔧 refactor: Enhance resolveHeaders to support user and custom variables

- Updated resolveHeaders function to accept user and custom user variables for placeholder replacement.
- Modified header resolution in multiple client and controller files to utilize the enhanced resolveHeaders functionality.
- Added comprehensive tests for resolveHeaders to ensure correct processing of user and custom variables.

* 🔧 fix: Update user ID placeholder processing in env.ts

* 🔧 fix: Remove arguments passing this.user rather than req.user

- Updated multiple client and controller files to call resolveHeaders without the user parameter

* 🔧 refactor: Enhance processUserPlaceholders to be more readable / less nested

* 🔧 refactor: Update processUserPlaceholders to pass all tests in mpc.spec.ts and env.spec.ts

* chore: remove legacy ChatGPTClient

* chore: remove LLM initialization code

* chore: initial deprecation removal of `gptPlugins`

* chore: remove cohere-ai dependency from package.json and package-lock.json

* chore: update brace-expansion to version 2.0.2 and add license information

* chore: remove PluginsClient test file

* chore: remove legacy

* ci: remove deprecated sendMessage/getCompletion/chatCompletion tests

---------

Co-authored-by: Dustin Healy <54083382+dustinhealy@users.noreply.github.com>

api/server/services/Endpoints/openAI/initialize.js

@@ -1,11 +1,7 @@
-const {
-  ErrorTypes,
-  EModelEndpoint,
-  resolveHeaders,
-  mapModelToAzureConfig,
-} = require('librechat-data-provider');
+const { ErrorTypes, EModelEndpoint, mapModelToAzureConfig } = require('librechat-data-provider');
 const {
   isEnabled,
+  resolveHeaders,
   isUserProvided,
   getOpenAIConfig,
   getAzureCredentials,
@@ -84,7 +80,10 @@ const initializeClient = async ({
     });
 
     clientOptions.reverseProxyUrl = baseURL ?? clientOptions.reverseProxyUrl;
-    clientOptions.headers = resolveHeaders({ ...headers, ...(clientOptions.headers ?? {}) });
+    clientOptions.headers = resolveHeaders(
+      { ...headers, ...(clientOptions.headers ?? {}) },
+      req.user,
+    );
 
     clientOptions.titleConvo = azureConfig.titleConvo;
     clientOptions.titleModel = azureConfig.titleModel;