test(cloudflare): stop comparing call-order timing halves in perf regression test

[gfargo]

Summary

• The "should maintain consistent performance across multiple operations" test compared the average duration of the first 5 vs. last 5 of 10 consecutive fetchConfig() calls, failing if the second half was >3x slower.
• This was flaky in a way that plain re-tuning the threshold wouldn't fix: repeated local runs (8 isolated runs, full durations logged) show the first 2-3 calls are consistently ~100x faster than the rest — interpreter/GC warm-up in the benchmark loop itself, not fetchConfig(). Whichever half absorbed that warm-up boundary swung its average unpredictably, so the ratio assertion passed or failed close to at random depending on system load.
• Tried switching to a median-based comparison (the natural "make it statistical" fix) — it's worse, not better: 10-27x ratio on every single run, because the warm-up split confounds any first-half/second-half comparison regardless of which statistic is used. No threshold or statistic fixes a structural confound in the benchmark design.
• fetchConfig() has no accumulating per-call state that could cause a real slowdown across repeated calls (confirmed by reading CloudflareFirewallService/CloudflareClient — its internal ApiCache would make repeat calls faster, not slower).
• Replaced the relative comparison with a per-call absolute ceiling (reusing the existing 10s fetch SLA constant already used elsewhere in this file), which still catches a real regression (a hang, or an accidental O(n²) over repeated calls) without being sensitive to warm-up noise.

Test plan

• Target test run standalone 10x in a row — 10/10 pass
• Full test file run 8x in a row — 19/19 tests pass each time
• Full cloudflare/__tests__/ directory run 3x in a row — 506/506 tests pass each time
• --ci mode run once — 19/19 pass
• eslint on the changed file — 0 errors (pre-existing no-console warnings only, not introduced by this change)
• tsc --noEmit — clean

[github-actions]

🎉 This PR is included in version 3.14.2 🎉

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• npm package (@latest dist-tag)
• GitHub release

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