fix(mesi): replace timing-dependent test assertions with deterministic synchronization (#116)

[s2x]

Summary

Fixes #116 — removes all timing-dependent assertions (elapsed > N, time.Sleep) from the mesi test suite and replaces them with deterministic channel-based synchronization.

Changes

mesi/include.go — Added rng func(int) int parameter to selectUrl(). When nil, falls back to rand.IntN. Enables deterministic RNG injection in tests.

mesi/fetchUrl_test.go — Fixed 4 tests:

• TestMESIParseContextPropagation — replaced elapsed > 2s with handlerReached channel (handler must NOT be reached when context is pre-cancelled)
• TestMESIParseContextCancellationStopsAllGoroutines — same pattern
• TestMESIParseContextCancellationMidParse — replaced time.Sleep(100ms) + <-time.After(2s) with <-handlerReached -> cancel() -> <-done
• TestFetchConcurrentContextCancellation — replaced time.Sleep(5s) + elapsed > 1s with select on r.Context().Done()

mesi/include_test.go — Fixed TestSelectUrlChoosesBasedOnRatio:

• Replaced statistical 10000-iteration 65-95% range check with 4 deterministic subtests using injected RNG

mesi/parser_test.go — Fixed TestMESIParseRespectsTimeout:

• Replaced time.Sleep(500ms) with select on r.Context().Done() + handlerBlock channel

Verification

• go vet ./... — clean
• go test -race ./... — all pass (mesi + middleware)