feat: display description of prompt as placeholder text when user types /{prompt} in chat input

[bryanchen-d]

display description of prompt as placeholder text when user types /{prompt} in chat input

Fixes: #269237

[Copilot]

Pull Request Overview

This PR adds functionality to display prompt descriptions as placeholder text when users type /prompt commands in the chat input. The feature enhances user experience by providing contextual help about what each prompt does directly in the input field.

Key Changes

• Added prompt description resolution and caching system
• Implemented placeholder text display for prompt slash commands
• Added event listener to refresh cached descriptions when prompts change

[Copilot]

The boolean expression is complex and hard to read. Consider extracting the predicate function or breaking it into multiple conditions with descriptive variable names.

Suggested change

	const onlyPromptCommandAndWhitespace = slashPromptPart && parsedRequest.every(p => p instanceof ChatRequestTextPart && !p.text.trim().length || p instanceof ChatRequestSlashPromptPart);
	function isWhitespaceOrPromptPart(p: IParsedChatRequestPart): boolean {
	return (p instanceof ChatRequestTextPart && !p.text.trim().length) || (p instanceof ChatRequestSlashPromptPart);
	}
	const onlyPromptCommandAndWhitespace = slashPromptPart && parsedRequest.every(isWhitespaceOrPromptPart);

[Copilot]

The async call to resolvePromptDescription is made without awaiting or handling potential errors, which could lead to unhandled promise rejections. Consider adding error handling or making this a fire-and-forget operation with explicit .catch().

Suggested change

	this.resolvePromptDescription(slashPromptPart);
	this.resolvePromptDescription(slashPromptPart).catch(err => {
	console.error('Failed to resolve prompt description:', err);
	});