✨ feat: Add Dynamic User Field Placeholder Support in MCP Variables (#7825)

* chore: linting in mcp.spec.ts

* chore: linting in mcp.ts

* feat(mcp): support dynamic user field placeholders in MCP environment variables

- Added user object handling in MCP options, allowing for dynamic user field processing in environment variables, headers, and URLs.
- Updated `processMCPEnv` to utilize user fields for more flexible configurations.

* chore: update backend review workflow to include unit tests for @librechat/data-schemas

api/server/controllers/agents/client.js

@@ -671,6 +671,7 @@ 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,
+          user: this.options.req.user,
         },
         recursionLimit: agentsEConfig?.recursionLimit,
         signal: abortController.signal,