feat: per-instance random source for deterministic behaviour#8
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Add WithRandomSource(src rand.Source) to allow per-instance control of the random sampling used to make shed/pass decisions. The source must be safe for concurrent use if the throttle is used concurrently. Follows the same pattern as WithNow and WithRejectedErrorFunc. Switch the package from math/rand to math/rand/v2. Update tests to use two fixed sources — alwaysShed (Uint64 returns 0) and neverShed (Uint64 returns MaxUint64) — replacing probabilistic assertions with exact ones throughout.
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Summary
WithRandomSource(src rand.Source)— new per-instance option that controls the random draw used to decide whether a request is shed. Follows the same pattern asWithNowandWithRejectedErrorFunc. Falls back to the globalrand.Float64()when not set. The source must be safe for concurrent use if the throttle is used concurrently (same contract as the rest of the API — no implicit wrapping).math/rand→math/rand/v2— the package now uses the v2 API, available since Go 1.22 (our minimum version).Deterministic tests — two fixed sources (
alwaysShed,neverShed) replace every probabilistic assertion in the test suite. Tests that previously relied on "shed rate should be > 30%" or "P(all forwarded) < 0.01%" now assert exact outcomes. Also replaces the two real-time integration tests (TestAdaptiveThrottleRecovery,TestAdaptiveThrottleMinimumRate) with clock- and formula-based equivalents, and addsTestRejectionProbabilityFormulaandTestRejectionProbabilityPriorityCrossContaminationto verify the formulas directly.