VizPanel: Allow clearing default values

[bfmatei]

grafana/grafana#91600

This PR allows clearing default values in panel options. Currently trying to remove a default value would result in no actual change.

A little explanation about the code:

• the logic to allow clearing out values was already there but it was overwritten by the merge with the getPanelOptionsWithDefaults function
• previously onOptionsChange was accepting isAfterPluginChange as a parameter
  • this parameter was not doing anything given that, in getPanelOptionsWithDefaults where it was used, it was not affecting options but fieldConfig. This was removed
  • the logic for this was moved to the actual plugin change logic
  • getPanelOptionsWithDefaults call was removed completely from onOptionsChange

📦 Published PR as canary version: 6.40.0--canary.1255.18001595104.0

✨ Test out this PR locally via:

npm install @grafana/[email protected]
npm install @grafana/[email protected]
# or 
yarn add @grafana/[email protected]
yarn add @grafana/[email protected]

[Copilot]

Pull Request Overview

This PR enables clearing default values in VizPanel options by refactoring the options merging logic. Previously, attempts to clear default values had no effect because the merge with default values would override the cleared state.

• Removed the isAfterPluginChange parameter from onOptionsChange as it wasn't being used correctly
• Moved the getPanelOptionsWithDefaults call from onOptionsChange to the plugin change logic where it's actually needed
• Simplified the options change handling to preserve cleared values instead of always merging with defaults

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[torkelo]

should we update unit test for this scenario?

[bfmatei]

@torkelo - absolutely! Fixed the test and added an additional one.