perf(browserless): clear per-request cleanup timers

[Kikobeats]

Note

Low Risk
Low risk: change is limited to per-request timeout cleanup in withPage, with added test coverage; main risk is altering timing behavior of forced page closure under long-running requests.

Overview
Prevents lingering per-request cleanup timers in withPage by keeping a handle to the setTimeout that force-closes the page, safely unref-ing it when available, and always clearTimeout-ing it in a finally block after success/error.

Adds a regression test that verifies these closePage cleanup timers are cleared after a successful withPage run by instrumenting setTimeout/clearTimeout and asserting all created timers are cleared.

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