core(gather): handle missing DOM resolve objects

[KirtiRamchandani]

Problem

• The AnchorElements gatherer can crash with "Cannot read properties of undefined (reading 'objectId')" when Chrome returns a DOM.resolveNode response without a Runtime remote object for an anchor node.

Root cause

• The shared DOM resolver destructured resolveNodeResponse.object.objectId unconditionally. Existing missing-node handling only covered protocol errors, not a successful-but-empty resolve response.

Solution

• Treat a missing Runtime object the same as other missing node cases by returning undefined from both DOM resolver helpers.
• Add regression coverage for backend-node and DevTools-node-path resolution responses without objects.

Tests run

• corepack yarn mocha --testMatch core/test/gather/driver/dom-test.js
• .\node_modules\.bin\eslint.cmd --quiet core\gather\driver\dom.js core\test\gather\driver\dom-test.js
• git diff --check

Fixes #16860