fix(text): interpolate token value in _normalize_token error message

[erensh27]

Fix for #658: the error message in _normalize_token() used a plain string literal with {token!r}, so the placeholder was printed literally instead of the offending token value.

# before
raise ValueError(
    'Invalid token: {token!r}. `stop_token`s and `forbidden_token`s must'
    ' map to single token ids in the vocab.'
)
# after
raise ValueError(
    f'Invalid token: {token!r}. `stop_token`s and `forbidden_token`s must'
    ' map to single token ids in the vocab.'
)

Now the exception reports the actual token (e.g. 'hello world') instead of the raw placeholder, making stop_tokens / forbidden_tokens misconfigurations debuggable.

Fixes #658

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[erensh27]

CLA check done