test(NODE-7831): widen duration bounds in flaky timing assertions

[tadjik1]

Description

Adds small timing tolerances to two flaky duration assertions (sleep timers run on a millisecond-granularity clock and can complete marginally early when measured with performance.now()).

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• Lint is passing (npm run check:lint)
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[Copilot]

🟢 Approval recommended

The remaining documentation nit is non-blocking.

Pull request overview

Widen timing assertion tolerances in CSFLE and client backpressure integration tests to reduce flaky failures.

Changes:

• Lower the CSOT timeout bound from 498ms to 495ms.
• Add 5ms tolerance to backoff duration assertions.
• Document timing precision rationale.

File summaries

File	Description
test/integration/client-side-encryption/driver.test.ts	Relaxes CSOT timeout timing bounds.
test/integration/client-backpressure/client-backpressure.prose.test.ts	Adds tolerance for backoff timing measurements.

Review details

Suppressed comments (1)

test/integration/client-backpressure/client-backpressure.prose.test.ts:220

• This explanation says the duration is measured with a higher-resolution clock, but measureDuration calls processTimeMS(), which floors performance.now() to whole milliseconds (test/tools/utils.ts:535-539, src/utils.ts:441-443). Please describe the actual millisecond-truncated measurement so the rationale for this tolerance is not misleading.

      //     are `0.1 + 0.2 = 0.3s`. The lower bounds include a small tolerance because sleep
      //     timers run on a millisecond-granularity clock, so a sleep can complete marginally
      //     earlier than the requested duration when measured with a higher-resolution clock.

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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