fix: Better loading state for Client Sessions page

[ernestii]

Address this issue:

when I visit sessions page and select the source, the instruction shows up and I can see sessions once they are loaded.

Improves loading state handling and checks for source type.

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Latest commit: 178c488

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Name	Status	Preview	Comments	Updated (UTC)
hyperdx-v2-oss-app	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 30, 2025 4:26pm

[Copilot]

Pull Request Overview

This PR refines the loading and validation state on the Client Sessions page so that users see a more responsive spinner and appropriate messaging based on both sessions and source loading states.

• Added a new isPending flag (isSessionSourceLoading) to the useSource hook.
• Updated conditional rendering to show a spinner if either sessions or session source are loading and refactored the UI messaging for invalid session sources and empty sessions.