Test suites for `Page.onResponse`
Add focused coverage for response callbacks: shared live Responses, lazy body retrieval, child-session cleanup, protocol header preservation, and TypeScript/Python SDK callback delivery and unsubscribe. A Chromium integration test exercises navigation, fetch, body retrieval, and unsubscribe; register it in the integration runner.
Part 2 of the page.onResponse stack. Targets zhao/page-on-response (#3356), so this diff contains only tests and their runner registration. Documentation and the release changeset follow in a separate child PR.
Validation: 262 TypeScript/docs/parity tests, 69 Python tests, the Chromium callback integration test, and extension/SDK TypeScript checks passed on the complete stack. The integration-runner suite also passed again after the split (13 tests).