fix: ModelRegistryClient ignored explicit URL ports

[a-reda-ok]

What this PR does / why we need it:

Fixes ModelRegistryClient port inference so URLs with an explicit port (for example, https://example.org:456 or http://example.org:456) correctly use that port when port is not passed as an argument.

Previously, initialization only inferred port from scheme (443 for HTTPS, 8080 for HTTP), which silently ignored explicit URL ports and could route clients to the wrong endpoint. The fix parses base_url with urlsplit and prefers parsed_url.port before falling back to scheme defaults. This preserves existing behavior for URLs without explicit ports while making documented explicit-port behavior actually work.

Regression coverage was updated in model_registry_client_test.py so both explicit HTTP/HTTPS port cases now assert port=456, validating the corrected behavior.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):

Fixes #

Checklist:

• Docs included if any changes are user facing

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