fix: PartitionOptions hash_rate upper bound to match u8 storage

[VolodymyrBg]

• Problem: The PartitionOptions.hash_rate field is stored as u8 but the constructor allowed values up to 256. Passing 256 truncates to 0 when cast to u8, is serialized as 0, and on deserialization becomes 0, which violates the invariant (>= 1) and can trigger a panic in PartitionOptions::new. The docstring for with_partitions also incorrectly stated the upper bound as 256.
• Why it’s a problem: u8 cannot represent 256; truncation silently corrupts the value, causing inconsistent serialization/deserialization behavior and potential runtime panics. This also breaks expectations for stable encoding of options.
• Fix: Tighten the hash_rate upper bound to 255 in PartitionOptions::new and update the with_partitions documentation accordingly. This prevents truncation, ensures values round-trip correctly via write_u8/read_u8, and keeps binary compatibility without widening the serialized type.

[meta-cla]

Hi @VolodymyrBg!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at [email protected]. Thanks!

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!