(protobuf) Support `StreamReadConstraints.maxDocumentLength` in `ProtobufParser`

`ProtobufParser` does not apply `StreamReadConstraints.maxDocumentLength` — neither for streaming (`InputStream`) input when reloading its buffer, nor up front for fixed `byte[]` input. CBOR and Smile both do (see #649 / [core#1548] for the `byte[]` case).

Add `validateDocumentLength()` calls in `loadMore()` / `_loadToHaveAtLeast()` and in `ProtobufFactory._createParser(..., byte[], int, int)`, matching the CBOR implementation.