gguf user defined tokens (#49008)
* GGUF tokenizer: keep the file's user-defined tokens whole
A GGUF marks tokens like `<think>`, `</think>`, `<tool_call>` and `<tool_response>` as
USER_DEFINED. llama.cpp always matches these whole in the text, and the reference tokenizers
carry them as non-special added tokens. The GGUF-built tokenizer only registered CONTROL
tokens, so `<think>` was cut into `<`, `think`, `>`: every chat prompt for Qwen3.5 differed
from what llama.cpp feeds the model. Add them as plain added tokens, restricted to spellings
already in the vocabulary so a sentencepiece file's raw whitespace runs do not grow it.
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