🏷️ 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/UserConnectionManager.ts

@@ -7,6 +7,7 @@ import type { MCPOAuthTokens } from '~/mcp/oauth';
 import { MCPConnectionFactory } from '~/mcp/MCPConnectionFactory';
 import { MCPServersRegistry } from '~/mcp/MCPServersRegistry';
 import { MCPConnection } from './connection';
+import type { RequestBody } from '~/types';
 import type * as t from './types';
 
 /**
@@ -47,6 +48,7 @@ export abstract class UserConnectionManager {
     serverName,
     flowManager,
     customUserVars,
+    requestBody,
     tokenMethods,
     oauthStart,
     oauthEnd,
@@ -57,6 +59,7 @@ export abstract class UserConnectionManager {
     serverName: string;
     flowManager: FlowStateManager<MCPOAuthTokens | null>;
     customUserVars?: Record<string, string>;
+    requestBody?: RequestBody;
     tokenMethods?: TokenMethods;
     oauthStart?: (authURL: string) => Promise<void>;
     oauthEnd?: () => Promise<void>;
@@ -127,6 +130,7 @@ export abstract class UserConnectionManager {
           oauthStart: oauthStart,
           oauthEnd: oauthEnd,
           returnOnOAuth: returnOnOAuth,
+          requestBody: requestBody,
         },
       );