fix: lower lookbehind size to needleLength - 1

[gurgunday]

lookbehind size doesn't need to be the same size as the needle, the last index is never used

Here's why: if it reaches the needle size with an incoming buffer, we immediately check and empty it anyway, we never store data that has enough length to be compared to the needle, it always has to be at max 1 less, otherwise the check would happen -- it's one of the guarantees of the algorithm

The test changes break nothing, we just had to change lookbehind length in expected checks

[gurgunday]

Bench (node 22):

master:

gurgunday@DELL-G15:~/work/OSS/busboy/benchmarks$ npm run benchmark-fastify

> busboy-benchmarks@1.0.0 benchmark-fastify
> node busboy/executioner.js -c 1

Mean time for fastify-busboy is 177610.58582281295 nanoseconds
all done

pr:

gurgunday@DELL-G15:~/work/OSS/busboy/benchmarks$ npm run benchmark-fastify

> busboy-benchmarks@1.0.0 benchmark-fastify
> node busboy/executioner.js -c 1

Mean time for fastify-busboy is 174747.183856004 nanoseconds
all done

Copilot reviewed 2 out of 2 changed files in this pull request and generated no suggestions.