DoApiRequest previously called io.ReadAll on every request body to apply
passthrough model mapping, even when the feature was disabled. This
caused unnecessary memory pressure for large request bodies (long
conversations, multipart image uploads).

Split the guard logic into needsPassthroughModelMapping() so that
ReadAll is only called when all conditions are met (feature enabled +
passthrough mode + model mapped). Non-passthrough requests now pass
the original io.Reader directly to http.NewRequest without buffering.

Rewrite tests to verify combined guard + modify behavior.