[BUGFIX] fix race condition with initial query plugin spec

[andreasgerstmayr]

Description

useDefaultQueryDefinition() returned isLoading: false while the plugin was still loading (only the metadata loading state was checked). This caused the initial plugin spec to be set to {} because defaultQueryPlugin was undefined.

Fix by combining both the metadata and plugin loading states into a single composite isLoading flag.

Screenshots

no visible UI changes

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.
• E2E tests are stable and unlikely to be flaky.
  See e2e docs for more details. Common issues include:
  • Is the data inconsistent? You need to mock API requests.
  • Does the time change? You need to use consistent time values or mock time utilities.
  • Does it have loading states? You need to wait for loading to complete.