fix: cap number of public reviews per milestone (#808)

[chiscookeke11]

Summary

I added a MAX_PUBLIC_REVIEWS_PER_MILESTONE cap (50) to open_review::submit_review to prevent unbounded review list growth per milestone and protect gas/CPU costs on ledger state updates.

Scope

I only worked on issue #808 (open_review::submit_review unbounded review list growth). I am not claiming any other issues in this PR.

Changes

• Added MAX_PUBLIC_REVIEWS_PER_MILESTONE constant (50) in constants.rs.
• Added TooManyPublicReviews error variant to ContractError in errors.rs.
• Updated submit_review in open_review.rs to reject new reviewers exceeding the cap while allowing existing reviewers to update their entries.
• Added unit test submit_review_rejects_when_cap_reached in open_review.rs.

Test Plan

• Ran cargo test --lib and verified all 165 tests pass (including submit_review_rejects_when_cap_reached).
• Ran cargo fmt to ensure clean formatting.

Closes #808

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