🏷️ 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/app/clients/BaseClient.js

@@ -37,6 +37,8 @@ class BaseClient {
     this.conversationId;
     /** @type {string} */
     this.responseMessageId;
+    /** @type {string} */
+    this.parentMessageId;
     /** @type {TAttachment[]} */
     this.attachments;
     /** The key for the usage object's input tokens
@@ -614,15 +616,19 @@ class BaseClient {
       this.currentMessages.push(userMessage);
     }
 
+    /**
+     * When the userMessage is pushed to currentMessages, the parentMessage is the userMessageId.
+     * this only matters when buildMessages is utilizing the parentMessageId, and may vary on implementation
+     */
+    const parentMessageId = isEdited ? head : userMessage.messageId;
+    this.parentMessageId = parentMessageId;
     let {
       prompt: payload,
       tokenCountMap,
       promptTokens,
     } = await this.buildMessages(
       this.currentMessages,
-      // When the userMessage is pushed to currentMessages, the parentMessage is the userMessageId.
-      // this only matters when buildMessages is utilizing the parentMessageId, and may vary on implementation
-      isEdited ? head : userMessage.messageId,
+      parentMessageId,
       this.getBuildMessagesOptions(opts),
       opts,
     );