starknet_patricia: change test util random u256

[dorimedini-starkware]

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[reviewable-StarkWare]

This change is 

[dorimedini-starkware]

• starknet_os: import random EC point logic #14195
• release: upgrade rand crate version #14194
• starknet_patricia: change test util random u256 #14193 👈 (View in Graphite)
• main

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• Started at 2026-05-25T13:38:58Z
• Started at 2026-05-25T14:50:36Z

[cursor]

PR Summary

Low Risk
Test-only utility and dependency cleanup; production Patricia logic is unchanged aside from updated test expectations.

Overview
Reimplements get_random_u256 in Patricia test helpers using ethnum::U256 and rejection sampling (with a bit-length mask) instead of num-bigint, and drops num-bigint from starknet_patricia dependencies and its testing feature.

Because random state-diff generation still uses a fixed seed but the sampling algorithm changed, the random-case committer integration test’s expected contracts trie root was updated accordingly. starknet_os dev-dependencies now pull num-bigint from the workspace without enabling rand on that dev-dependency directly (the testing feature still wires num-bigint/rand where needed).

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[nimrod-starkware]

@nimrod-starkware reviewed 4 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on ArielElp and yoavGrs).

[nimrod-starkware]

@nimrod-starkware reviewed 1 file and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on ArielElp and yoavGrs).