Reject a zero length in the FixedLength pre-tokenizer instead of panicking

[v-code01]

What

FixedLength::length is deserialized from the config with no validation (FixedLength::new performs none either). A value of 0 reaches:

for chunk in char_positions.chunks(self.length) { // panics: "chunk size must be non-zero"

so a crafted {"type":"FixedLength","length":0} pre-tokenizer crashes tokenization on any non-empty input.

Fix

Reject length == 0 with a clear error at the start of pre_tokenize:

if self.length == 0 {
    return Err("FixedLength pre-tokenizer length must be greater than 0".into());
}

Tests

Adds zero_length_is_rejected; existing FixedLength tests are unchanged.

cc @ArthurZucker @McPatate