Apply batched session and browser configuration plus optional navigation

Optional multipart parts apply browser and session configuration in one serialized request. Profiles, nonempty policies, nonempty flags, and extensions use one Chromium stop/apply/start cycle by default. Display-only and start_url-only requests stay live. Set extension_load_strategy=prefer_cdp to attempt live CDP activation when extensions are present without a profile, nonempty policies, or nonempty flags. Enterprise-policy extensions and CDP activation failures fall back to one restart within the same request. The query parameter is ignored by older images, which retain the default restart behavior. Optional start_url dispatches a best-effort navigation after configuration and readiness without waiting for page load. Bare hosts are normalized to https://. Omit any part you do not need. At least one actionable part must be present. Required configuration steps run in this order: policies, extensions, display, flags, then profile archive. The endpoint is not transactional: if a later required step fails, earlier successful side effects may remain on the instance while the non-2xx error propagates. Prefer this over separate /chromium/* and /display calls when multiple restart-triggering steps apply in one session configure.