fix(vrf): replace mutable default RNG with per-call generator

[Sertug17]

Fixes #1733

Summary

The previous signature used rng=np.random.default_rng(seed=int(datetime.now().timestamp())) as a default argument evaluated once at module import time, shared across every call. This made validator selection entirely predictable after server startup.

Changes

• backend/consensus/vrf.py rng default changed to None, fresh np.random.default_rng() created per call inside the function body. Unused datetime import removed.
• tests/unit/test_vrf.py regression test verifies 200 independent calls produce varied results

Test plan

• pytest tests/unit/test_vrf.py passes including new regression test

[github-actions]

This PR targeted main, which is only the default/static branch.

I retargeted it to v0.123-dev, the active development branch. Pushes to v0.123-dev automatically fast-forward main.

[coderabbitai]

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