MultiModelRead returns PDF bytes directly, which model cannot recognize

adk/backend/Local.MultiModalRead 模型没有传 pages 参数时, 直接返回了PDF二进制数据，模型无法识别
```go
if !paged {
return &filesystem.MultiFileContent{
Parts: []filesystem.FileContentPart{
{
Type: filesystem.FileContentPartTypePDF,
MIMEType: "application/pdf",
Data: data,
},
},
}, nil
}
```

Start in adk/backend/Local.MultiModalRead and inspect the no-pages branch alongside the paged branch. Trace how MultiFileContent PDF parts are passed to the model, then verify that the returned content is recognized as intended when pages is omitted.

Written by the indexing model from the issue text.