fix: validate ngram_size in get_repetition_penalty_reward

[CJstate]

What does this PR do?

Adds construction-time validation for ngram_size in get_repetition_penalty_reward. Zero or negative values were accepted at construction time but caused a ZeroDivisionError (or silently produced empty n-grams) during reward computation, as reported in #7015.

The fix adds a check consistent with the existing max_penalty validation:

if ngram_size <= 0:
    raise ValueError(f"ngram_size {ngram_size} should be greater than 0")

Also adds a parametrized regression test covering ngram_size=0 and ngram_size=-1.

Fixes #7015

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case. (get_repetition_penalty_reward accepts non-positive ngram_size #7015)
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

AI writing disclosure

• AI-assisted: some parts were suggested or improved by AI, but the PR was written and reviewed by a human.

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR.

---

Note

Low Risk
Small input-validation change in a reward helper with regression tests; no auth, data, or training-path behavior changes beyond failing fast on invalid config.

Overview
get_repetition_penalty_reward now rejects non-positive ngram_size at construction with a ValueError, matching the existing max_penalty guard. Previously, 0 or negative values could slip through and fail during scoring (e.g. ZeroDivisionError when ngram_size=0).

A parametrized test in tests/test_rewards.py asserts that ngram_size 0 and -1 raise ValueError.

^{Reviewed by Cursor Bugbot for commit 4eec16b. Bugbot is set up for automated code reviews on this repo. Configure here.}

[albertvillanova]

Duplicate of:

• Validate repetition penalty n-gram size #7016