Fix baseUrl subpath being ignored when constructing API URLs

Brian Perryrequested to merge
issue/api_client-3491907:base-url into canary

Normalize baseUrl to always end with a trailing slash in the ApiClient constructor, and remove leading slashes from relative URL segments passed to new URL(). Previously, a baseUrl like http://host/path would be ignored by the URL constructor when the relative path started with "/", causing requests to go to http://host/jsonapi instead of http://host/path/jsonapi.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

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