Display invalid interpreter errors as modal dialogs

[Copilot]

Invalid interpreter errors now appear as modal dialogs instead of dismissible notifications, increasing visibility for blocking errors when running files, starting REPL, or debugging.

Changes

• Modified DiagnosticCommandPromptHandlerService.displayMessage() to pass { modal: true } for all error-severity diagnostics
• Updated unit tests to verify modal option is correctly applied to error messages while warnings/info remain non-modal

Implementation

// Before
return this.appShell.showErrorMessage(message, ...prompts);

// After
return this.appShell.showErrorMessage(message, { modal: true }, ...prompts);

This applies to all error-level diagnostics including InvalidPythonInterpreterDiagnostic and NoPythonInterpretersDiagnostic, ensuring users must acknowledge blocking errors before continuing.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Change "invalid interpreter" notification to a modal dialog </issue_title>
<issue_description>
Right now when there's an invalid interpreter selected for the workspace and you try to run a file or start the REPL, for example, we display an error notification. Feedback from the broader team was that we should probably change it to a modal dialog since it's an "blocking" error, to drive higher discoverability of the failure point. </issue_description>

Comments on the Issue (you are @copilot in this section)

@luabud I think this is an example of a modal dialog in the Jupyter extension: https://github.com/microsoft/vscode-jupyter/blob/main/src/notebooks/controllers/commands/installPythonControllerCommands.ts#L52

• Fixes Change "invalid interpreter" notification to a modal dialog  #23432

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