🏷️ 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>

api/server/controllers/agents/client.js

@@ -768,6 +768,11 @@ class AgentClient extends BaseClient {
           last_agent_index: this.agentConfigs?.size ?? 0,
           user_id: this.user ?? this.options.req.user?.id,
           hide_sequential_outputs: this.options.agent.hide_sequential_outputs,
+          requestBody: {
+            messageId: this.responseMessageId,
+            conversationId: this.conversationId,
+            parentMessageId: this.parentMessageId,
+          },
           user: this.options.req.user,
         },
         recursionLimit: agentsEConfig?.recursionLimit ?? 25,