[Gladorme]

Description

Split out of #319 to ease review (independent from the other split PRs, based on main).

Replace the useEffect that wrote the first explorer plugin into the store with a derived default.

• ViewExploreApp derives the default explorer from the sorted plugin list and passes it as the new optional defaultExplorer prop of ExplorerManagerProvider / ExplorerManagerProviderWithQueryParams. It is used while the store has no explorer selected and committed to the store on the first state change.
• New exported helpers getExplorerKey(plugin) and useSortedExplorerPlugins() (shared by the tabs and the default selection, so the default always matches the first tab).
• Avoids an extra render + URL update on first load of the Explore page.
• Added tests for ExploreManager and ExplorerManagerProvider.

Validation on this branch: npm run lint, npm run format:check, npm run type-check and npm run test all pass.

Screenshots

No visual change.

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.