[Testing] Restore behavioral bounds weakened by flakiness fixes

[janezpodhostnik]

Some of the flakiness fixes in #8634 removed assertions along with the flakiness. This restores the underlying behavioral guarantees in a form that cannot flake.

Changes

• websockets rate limiter: re-add an upper bound on the total stream duration (generous slack), so a limiter that stalls the stream fails the test again. The removed per-message +-5ms assertion is not restored, pair-wise delays are legitimately variable for a token bucket.
• ALSP decay: re-add a pacing assertion, reaching 3 heartbeats worth of decay must take at least 2 heartbeat intervals. Lower bound only, so it cannot fail on a slow machine.
• client timeout tests: the slow-handler path is exercised deterministically again. The handler signals receipt, and the client retries (each attempt must still time out) until at least one timeout was caused by the slow handler rather than by connection setup.
• RemoveTempDir: log when retries were needed, preserving the leaked-background-writer signal that the retry loop would otherwise hide.

Related: #8634

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Dependency Review

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[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
utils/unittest/unittest.go	0.00%	3 Missing ⚠️

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