FN-5858: fix multipart chat attachment sends

Handle multipart chat message requests so uploaded attachments reach the chat manager.

- parse multipart bodies on the chat message SSE endpoint with attachment count and size handling
- persist uploaded message attachments into session storage and forward generated metadata to sendMessage
- add multipart route coverage for successful sends, missing content validation, and the published package changeset

Files changed:
 .changeset/fn-5858-chat-multipart-fix.md           |  7 ++
 .../src/__tests__/chat-attachment-routes.test.ts   | 33 ++++++++
 .../dashboard/src/__tests__/chat-routes.test.ts    | 79 +++++++++++++++++++
 .../dashboard/src/routes/register-chat-routes.ts   | 92 +++++++++++++++-------
 4 files changed, 183 insertions(+), 28 deletions(-)

Fusion-Task-Id: FN-5858

Fusion-Task-Lineage: 3b1320a7-14ba-4699-8bfd-eba193a5de7e

.changeset/fn-5858-chat-multipart-fix.md

@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix chat message sends with file attachments by parsing multipart form bodies on the chat messages SSE endpoint.
+
+Uploaded message attachments are now validated, persisted to the session attachment directory, converted into chat attachment metadata, and forwarded to the chat manager while JSON-only message sends continue to work unchanged.