fix(api_client): update path handling for hosts with subpaths

[xbaotg]

Fix URL joining logic to ensure correct path resolution for API requests.

Previously, using urljoin could result in incorrect URLs if the host ended with a path and the formatted url started with a leading slash. For example, a host like https://abc.com/api/qdrant combined with url.format(**path_params) producing "/collections" would incorrectly resolve to "https://abc.com/collections".

To address this, the host is now stripped of any leading slashes and ensured to end with a single /, while the url is stripped of leading slashes. This ensures the resulting URL maintains the expected path hierarchy: Example fix: "https://abc.com/api/qdrant" + "/collections" -> "https://abc.com/api/qdrant/collections"

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[henninglebbaeus]

I would highly appreciate if this fix can be merged. This is behavior is blocking our Qdrant adoption, as all our URLs will contain path prefixes.