Fix: Keep Kick start your query button visible while typing in a new query editor

[henrybrewer00-dotcom]

Summary

The "Kick start your query" button in the Prometheus query editor disappears as soon as the user starts typing in a new editor. The button should only hide when the editor opens with an existing (saved) query.

• Capture Boolean(props.query.expr) on mount via a useRef and gate the button on that initial value.
• New editors keep the button visible regardless of typing; editors loaded with a saved query continue to hide it.
• Adds three unit tests covering the new behavior in PromQueryEditorSelector.test.tsx.

Fixes #108 (restores the intent of grafana/grafana#113762 without the regression).

Test plan

• jest — all 918 @grafana/prometheus tests pass, including 3 new cases in PromQueryEditorSelector.test.tsx:
  • shows the Kick start your query button for a new query editor
  • hides the Kick start your query button when the editor opens with an existing query
  • keeps the Kick start your query button visible after typing in a new query editor
• yarn typecheck passes
• Manual: open a new Prometheus query editor, type a partial query — button should remain visible. Open a saved dashboard panel with an existing Prometheus query — button should be hidden.

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[itsmylife]

@henrybrewer00-dotcom Thank you for your contribution. I'm reviewing it but I won't be able to merge it unless you sign your commits. Our CI dictates that all commits must be signed. Also you have to sign CLA: #87 (comment)

[github-actions]

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