[V1] Get vocab size from model config

[russellb]

This restores this line of code back to the original implementation.
We've changed it multiple times trying to work around xgrammar's
handling of additional special tokens, but 0.1.16 should be good with
this original behavior.

The fix in particular that simplifies this for us is:

mlc-ai/xgrammar@70c959f

Signed-off-by: Russell Bryant [email protected]

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[russellb]

xgrammar API changes broke the V0 integration ... we can hold this in case I want to add more to it tomorrow to test something. @aarnphm is also trying to resolve this in #14868. We will probably end up merging that one.