Stop a body-less response from killing the whole sitespeed run

[marcusosterberg]

chrome-custom.cjs enables CDP Fetch interception on Documents and then called Fetch.getResponseBody unconditionally. When the response has no retrievable body the call rejects with "Can only get response body on requests captured after headers received". The listener is async, so that rejection is unhandled, and under Node 24 an unhandled rejection terminates the process - which is the sitespeed process. One awkward response therefore lost the entire test run, not just that request.

Reproducer: any URL answering 401 with content-length 0, for example an Azure App Service behind Entra auth. Test 15 died after ~7 seconds and scored -1.

Fixes:

• Release requests paused before a response exists (network error, or no responseStatusCode); getResponseBody cannot succeed for those.
• Guard getResponseBody and fulfillRequest, releasing the request instead of leaking a rejection.
• Scope bodyResult with let. It was an implicit global assigned across an await, so concurrent Document interceptions could read each other's body.

Fetch.continueRequest is used to release in both stages; verified against Chromium 150 that it resolves a Response-stage pause (8/8). A pause that is never released would stall the page load, so the failure is logged rather than swallowed.

Verified: the 401 site now completes and scores instead of crashing. On a healthy site the new branches never execute and the rating is unchanged (4.49 vs 4.47, run-to-run variance). With a sitespeed-rules.json present, header and HTML rewriting still applies identically (2 header injections, 2 HTML rewrites, 0 crashes, before and after).