[go] support io.Reader response types in client decode

The Go generator emits *os.File for type: string / format: binary by
default; callers can override that with --type-mappings to get
io.Reader. The shared decode helper, however, only branches on *string,
*os.File and **os.File — when the generated Execute method declares
localVarReturnValue io.Reader, decode is called with v = *io.Reader,
none of the existing assertions match and the call fails with
"undefined response type".

Add an *io.Reader branch right after the *string branch (mirroring the
io.Reader-first preference in setBody) that wraps the already-buffered
response bytes in a bytes.Reader. Bytes are pre-slurped by the caller
for shared error/success handling, so a wrapping reader is the smallest
correct fix.

Regenerated the eleven affected Go samples.

Author: dschmidt

modules/openapi-generator/src/main/resources/go/client.mustache

@@ -539,6 +539,10 @@ func (c *APIClient) decode(v interface{}, b []byte, contentType string) (err err
 		*s = string(b)
 		return nil
 	}
+	if r, ok := v.(*io.Reader); ok {
+		*r = bytes.NewReader(b)
+		return nil
+	}
 	if f, ok := v.(*os.File); ok {
 		f, err = os.CreateTemp("", "HttpClientFile")
 		if err != nil {