Update ModifiedFilesConfirmationTool to handle missing selectedCustomButton

[DonJayamanne]

No description provided.

[Copilot]

Pull request overview

Updates the chat built-in ModifiedFilesConfirmationTool to no longer throw when selectedCustomButton is missing, returning a fallback value instead.

Changes:

• Removed the hard error when invocation.selectedCustomButton is undefined.
• Added a fallback string value in the tool result when no custom button label is present.

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/common/tools/builtinTools/confirmationTool.ts:258

• This change introduces a new behavior path (handling selectedCustomButton being undefined) but there is no corresponding assertion in the existing ModifiedFilesConfirmationTool tests. Add a unit test that covers the missing-selection case (e.g. when confirmation is auto-approved by a hook) and verifies the returned value matches the intended default option.

		return {
			content: [{
				kind: 'text',
				value: invocation.selectedCustomButton || 'The user chose to skip the tool call, they want to proceed without running it'
			}]