Both functions called the global fetch directly, bypassing the fetchImpl
stored in the client closure. This silently ignored any custom fetch
implementation passed to createMattermostClient (test mocks, proxy-aware
fetchers, SSRF guards).
Switch both to client.request<void>() which uses fetchImpl, auto-injects
the Authorization header, handles Content-Type for JSON bodies, and
propagates errors consistently with every other client function.
uploadMattermostFile retains its direct fetch call (multipart/form-data
conflicts with request's automatic Content-Type injection).
Addresses Greptile review: 'New functions bypass client.request,
ignoring custom fetchImpl'.
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