Resolve a legacy endpoint event URI (which may be absolute, root-relative,
or document-relative) against the GET-SSE base URL, yielding the absolute URL
to POST subsequent JSON-RPC messages to. An absolute URI is only accepted when
it is same-origin with the base; a cross-origin absolute URI yields null so the
legacy fallback fails closed rather than POSTing the bearer token off-origin.
Resolve a legacy endpoint event URI (which may be absolute, root-relative, or document-relative) against the GET-SSE base URL, yielding the absolute URL to POST subsequent JSON-RPC messages to. An absolute URI is only accepted when it is same-origin with the base; a cross-origin absolute URI yields null so the legacy fallback fails closed rather than POSTing the bearer token off-origin.