fix(browser): unflake TestPageOnResponse nil panic

[alirezaedalat]

Summary

• Stop launching an unused Go-level Chromium in TestPageOnResponse via withSkipLaunch(); the test only needs the HTTP server and the managed browser from StartIteration. Double launches contributed to flaky error building browser on IterStart: canceled under CI load.
• Harden k6test.ToPromise against a nil sobek.Value so interrupted event loops fail the test cleanly instead of panicking on Export() (the panic in TestPageOnResponse is flaky #5124).
• Switch the test to RunPromise and add a regression unit test for the nil-value path.

Fixes #5124

Test plan

• go test -run TestToPromiseNilValue ./internal/js/modules/k6/browser/k6ext/k6test/
• go test -run '^TestPageOnResponse$' ./internal/js/modules/k6/browser/tests/
• go test -count=10 -parallel 4 -run '^TestPageOnResponse$' ./internal/js/modules/k6/browser/tests/
• CI browser tests on this PR

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Commit	Author	Reason	Message
e1b4ea67	alirezaedalat	unsigned	fix(browser): unflake TestPageOnResponse nil panic

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