Fix missing post_processor in DebertaV2Tokenizer causing no special t… (#44570)
* Fix missing post_processor in DebertaV2Tokenizer causing no special tokens
In transformers v5, DebertaV2Tokenizer was rewritten to use
TokenizersBackend
but the post_processor (responsible for adding [CLS]/[SEP] tokens) was
not set.
This caused `add_special_tokens=True` to silently produce output
without
special tokens for models like microsoft/mdeberta-v3-base.
* remove test_add_special_tokens
* Add test for post_processor special tokens in DebertaV2Tokenizer