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Make ChaosPolicy Randomness Injectable for Deterministic Chaos Tests

Summary

This PR refactors ChaosPolicy in src/chaos/chaosPolicy.ts to make the random number generator injectable, enabling deterministic chaos testing and reproducible incident debugging.

Problem

ChaosPolicy.decide() directly called Math.random() when chaosMode === 'random', coupling the policy to a global, non-seedable RNG. This made probabilistic chaos testing impossible to reproduce and hard to debug when chaos-induced incidents occurred.

Solution

Added an optional random: () => number parameter to the ChaosPolicy constructor:

  • Defaults to Math.random for production behavior
  • Allows tests to supply deterministic RNG sequences
  • Enables exact reproduction of chaos decision sequences

Changes

src/chaos/chaosPolicy.ts

  1. Added RandomFn type alias for the random function signature
  2. Updated constructor to accept optional random parameter with Math.random default
  3. Added explicit boundary handling in random mode:
    • probability <= 0: always returns 'none' (never injects)
    • probability >= 1: always returns 'error' (always injects)
  4. Enhanced JSDoc with security notes about chaos being gated on chaosMode

src/chaos/chaosPolicy.test.ts

Added comprehensive tests for the injectable RNG:

  • Tests use injected functions instead of spying on Math.random
  • Deterministic sequence tests prove exact decision reproduction
  • Boundary tests for probability 0 and 1 (and edge cases like negative/greater-than-1)
  • Production behavior test confirms default Math.random works correctly

Test Coverage

All 23 tests pass with coverage focused on:

  • Mode dispatch (off, error, timeout, random, unknown)
  • Target matching (empty, specific, case-insensitive)
  • Probability boundaries (0, 1, values in between, edge cases)
  • Deterministic RNG injection and sequence reproduction

Security

  • Chaos can never be active in production by default (gated on chaosMode)
  • The Math.random default provides the same cryptographic-quality randomness as Node.js
  • No behavior change for production code paths

closes #465