👤 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/initializeMCP.js

@@ -1,9 +1,9 @@
 const { logger } = require('@librechat/data-schemas');
-const { CacheKeys, processMCPEnv } = require('librechat-data-provider');
+const { CacheKeys } = require('librechat-data-provider');
+const { findToken, updateToken, createToken, deleteTokens } = require('~/models');
 const { getMCPManager, getFlowStateManager } = require('~/config');
 const { getCachedTools, setCachedTools } = require('./Config');
 const { getLogStores } = require('~/cache');
-const { findToken, updateToken, createToken, deleteTokens } = require('~/models');
 
 /**
  * Initialize MCP servers
@@ -30,7 +30,6 @@ async function initializeMCP(app) {
         createToken,
         deleteTokens,
       },
-      processMCPEnv,
     });
 
     delete app.locals.mcpConfig;