🏷️ feat: Request Placeholders for Custom Endpoint & MCP Headers (#9095)

* feat: Add conversation ID support to custom endpoint headers

- Add LIBRECHAT_CONVERSATION_ID to customUserVars when provided
- Pass conversation ID to header resolution for dynamic headers
- Add comprehensive test coverage

Enables custom endpoints to access conversation context using {{LIBRECHAT_CONVERSATION_ID}} placeholder.

* fix: filter out unresolved placeholders from headers (thanks @MrunmayS)

* feat: add support for request body placeholders in custom endpoint headers

- Add {{LIBRECHAT_BODY_*}} placeholders for conversationId, parentMessageId, messageId
- Update tests to reflect new body placeholder functionality

* refactor resolveHeaders

* style: minor styling cleanup

* fix: type error in unit test

* feat: add body to other endpoints

* feat: add body for mcp tool calls

* chore: remove changes that unnecessarily increase scope after clarification of requirements

* refactor: move http.ts to packages/api and have RequestBody intersect with Express request body

* refactor: processMCPEnv now uses single object argument pattern

* refactor: update processMCPEnv to use 'options' parameter and align types across MCP connection classes

* feat: enhance MCP connection handling with dynamic request headers to pass request body fields

---------

Co-authored-by: Gopal Sharma <gopalsharma@gopal.sharma1>
Co-authored-by: s10gopal <36487439+s10gopal@users.noreply.github.com>
Co-authored-by: Dustin Healy <dustinhealy1@gmail.com>

packages/api/src/mcp/MCPConnectionFactory.ts

@@ -1,7 +1,6 @@
 import { logger } from '@librechat/data-schemas';
 import type { OAuthClientInformation } from '@modelcontextprotocol/sdk/shared/auth.js';
 import type { TokenMethods } from '@librechat/data-schemas';
-import type { TUser } from 'librechat-data-provider';
 import type { MCPOAuthTokens, MCPOAuthFlowMetadata } from '~/mcp/oauth';
 import type { FlowStateManager } from '~/flow/manager';
 import type { FlowMetadata } from '~/flow/types';
@@ -10,23 +9,6 @@ import { MCPTokenStorage, MCPOAuthHandler } from '~/mcp/oauth';
 import { MCPConnection } from './connection';
 import { processMCPEnv } from '~/utils';
 
-export interface BasicConnectionOptions {
-  serverName: string;
-  serverConfig: t.MCPOptions;
-}
-
-export interface OAuthConnectionOptions {
-  useOAuth: true;
-  user: TUser;
-  customUserVars?: Record<string, string>;
-  flowManager: FlowStateManager<MCPOAuthTokens | null>;
-  tokenMethods?: TokenMethods;
-  signal?: AbortSignal;
-  oauthStart?: (authURL: string) => Promise<void>;
-  oauthEnd?: () => Promise<void>;
-  returnOnOAuth?: boolean;
-}
-
 /**
  * Factory for creating MCP connections with optional OAuth authentication.
  * Handles OAuth flows, token management, and connection retry logic.
@@ -49,15 +31,20 @@ export class MCPConnectionFactory {
 
   /** Creates a new MCP connection with optional OAuth support */
   static async create(
-    basic: BasicConnectionOptions,
-    oauth?: OAuthConnectionOptions,
+    basic: t.BasicConnectionOptions,
+    oauth?: t.OAuthConnectionOptions,
   ): Promise<MCPConnection> {
     const factory = new this(basic, oauth);
     return factory.createConnection();
   }
 
-  protected constructor(basic: BasicConnectionOptions, oauth?: OAuthConnectionOptions) {
-    this.serverConfig = processMCPEnv(basic.serverConfig, oauth?.user, oauth?.customUserVars);
+  protected constructor(basic: t.BasicConnectionOptions, oauth?: t.OAuthConnectionOptions) {
+    this.serverConfig = processMCPEnv({
+      options: basic.serverConfig,
+      user: oauth?.user,
+      customUserVars: oauth?.customUserVars,
+      body: oauth?.requestBody,
+    });
     this.serverName = basic.serverName;
     this.useOAuth = !!oauth?.useOAuth;
     this.logPrefix = oauth?.user