* fix(gateway): preserve message additional_kwargs in normalize_input (#3132)
The gateway's hand-rolled dict→message coercion only forwarded `content`
and collapsed every role to `HumanMessage`, silently dropping the
frontend's `additional_kwargs.files` payload (along with `id`, `name`,
and ai/system/tool roles).
Effect on issue #3132:
- `UploadsMiddleware` saw no `files` on the last human message, so the
just-uploaded file got bucketed under "previous messages" while the
current turn was reported as `(empty)`.
- The persisted human message had no `files`, so the attachment chip on
the message disappeared the moment the optimistic UI cleared.
Delegate the conversion to `langchain_core.messages.utils.convert_to_messages`
so `additional_kwargs`, `id`, `name`, and non-human roles round-trip
unchanged.
* fix(gateway): convert malformed-message ValueError into HTTP 400
normalize_input now sits at the request boundary, so a malformed
input.messages[N] dict (missing role/type/content, unsupported role,
etc.) should surface as 400 with the offending index — not bubble out
of FastAPI as 500.
Per Copilot review on #3136.
9c03a71a07