👤 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>

packages/api/src/endpoints/openai/initialize.ts

@@ -1,9 +1,4 @@
-import {
-  ErrorTypes,
-  EModelEndpoint,
-  resolveHeaders,
-  mapModelToAzureConfig,
-} from 'librechat-data-provider';
+import { ErrorTypes, EModelEndpoint, mapModelToAzureConfig } from 'librechat-data-provider';
 import type {
   LLMConfigOptions,
   UserKeyValues,
@@ -13,6 +8,7 @@ import type {
 import { createHandleLLMNewToken } from '~/utils/generators';
 import { getAzureCredentials } from '~/utils/azure';
 import { isUserProvided } from '~/utils/common';
+import { resolveHeaders } from '~/utils/env';
 import { getOpenAIConfig } from './llm';
 
 /**
@@ -91,7 +87,10 @@ export const initializeOpenAI = async ({
     });
 
     clientOptions.reverseProxyUrl = configBaseURL ?? clientOptions.reverseProxyUrl;
-    clientOptions.headers = resolveHeaders({ ...headers, ...(clientOptions.headers ?? {}) });
+    clientOptions.headers = resolveHeaders(
+      { ...headers, ...(clientOptions.headers ?? {}) },
+      req.user,
+    );
 
     const groupName = modelGroupMap[modelName || '']?.group;
     if (groupName && groupMap[groupName]) {