fix(ui): allow Populate to use selectedHost when serviceURI is empty

[najeebkp]

Pull Request type

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• WHOSUSING.md
• Other (please describe):

NOTE: Please remember to run ./gradlew spotlessApply to fix any format violations.

Changes in this PR

Summary

• Fix: "Populate" button in the Populate from remote services modal silently did nothing when the user picked a Host from the dropdown but the underlying service had no serviceURI. The guard in handleExecute required serviceURI; it now accepts serviceURI || selectedHost.
• UX: button is now properly disabled when prerequisites are missing, with a tooltip explaining which field is needed (service / host / method / required params). HTTP shows "Select a host", gRPC shows "Select a host:port".
• Tests: added ServiceRegistrySelector.test.tsx (13 component tests) covering the guard logic, tooltip copy for HTTP/gRPC, and populate payload shape for both protocols.