docs(contributing): clarify event.fetch required even for multipart

The multipart note previously said "use raw fetch" which was misread
as "global fetch is acceptable". Clarify that event.fetch must always
be used — the typed client is bypassed for multipart, but handleFetch
still needs to inject the session cookie.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -263,7 +263,7 @@ if (!result.response.ok) {
 return { person: result.data! }; // non-null assertion is safe after the ok check
 ```
 
-For multipart/form-data (file uploads): bypass the typed client and use raw `fetch` — the client cannot handle it.
+For multipart/form-data (file uploads): bypass the typed client and use `event.fetch` directly — never global `fetch`. The typed client cannot handle multipart bodies, but `event.fetch` is still required so that `handleFetch` injects the session cookie.
 
 ### Date handling