🛟 fix: Keep File Uploads Alive With SSE Heartbeats (#14295)

* fix: Use SSE to upload files in order to avoid idle timeouts.  Idle timeouts can occur for example from gateways and other services like cloudfare when uploading large files.  For example during rag processing the file is uploaded to librechat which then sends it to rag.  While librechat is waiting for the embeddings to come back from rag the file upload is sitting idle.  Gateways tend to want to cancel the upload with an http 408 , 504, or 524.  This change uses SSE to perform the upload so that while librechat is sending the file to rag, it consistently sends back a heartbeat event to the client to keep the connection alive.  This is especially useful when utilizing  EMBEDDING_BATCH_SIZE in librechat rag which will allow rag to process signifigantly larger files without running out of memory.

* added tests to packages\api\src\files\sse.spec.ts in order to test the new sse.ts

* fix: Harden SSE file upload lifecycle

* style: Sort data provider imports

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Co-authored-by: Marc Amick <MarcAmick@jhu.edu>
Co-authored-by: Danny Avila <danny@librechat.ai>

.env.example

@@ -577,6 +577,9 @@ TTS_API_KEY=
 # EMBEDDINGS_PROVIDER=openai
 # EMBEDDINGS_MODEL=text-embedding-3-small
 
+# Stream upload responses with heartbeats during long-running file processing.
+# FILE_UPLOAD_SSE_ENABLED=false
+
 #===================================================#
 #                    User System                    #
 #===================================================#