feat(copilotcli): enhance usage reporting in CopilotCLISession with detailed token metrics

[DonJayamanne]

Fixes #300491

[Copilot]

Pull request overview

This PR improves token usage reporting for Copilot CLI chat sessions so the chat context-usage indicator can reflect more accurate prompt usage and show a breakdown of prompt token categories, addressing the reported “compaction doesn’t work” symptom.

Changes:

• Adds reporting of promptTokenDetails (system instructions, tool definitions, messages) alongside prompt/completion token counts.
• Listens for session.usage_info events to capture current token distribution and report it to the response stream.
• Attempts to refresh usage after each request by querying _sdkSession.usage.getMetrics().

Show a summary per file

File	Description
extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSession.ts	Adds usage reporting helpers, hooks session.usage_info, and builds prompt token breakdown for the context usage widget.

Copilot's findings

Comments suppressed due to low confidence (2)

extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSession.ts:585

• The session.usage_info handler assumes all token fields are numbers, but event.data appears to be untyped (similar to assistant.usage, which guards with typeof === 'number'). Please add type checks (at least for currentTokens) before assigning to lastUsageInfo to avoid propagating NaN/invalid values into the context usage widget.

			disposables.add(toDisposable(this._sdkSession.on('session.usage_info', (event) => {
				lastUsageInfo = {
					currentTokens: event.data.currentTokens,
					systemTokens: event.data.systemTokens,
					conversationTokens: event.data.conversationTokens,

extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSession.ts:590

• reportUsage(lastUsageInfo.currentTokens, 0) resets completionTokens to 0 whenever session.usage_info fires. The context usage widget computes used tokens as promptTokens + completionTokens, so this will under-report usage and can cause the indicator to jump backwards if this event arrives after assistant.usage. Consider preserving the latest known completion token count (or using metrics.lastCallOutputTokens) when reporting session.usage_info.

				reportUsage(lastUsageInfo.currentTokens, 0);
			})));

• Files reviewed: 1/1 changed files
• Comments generated: 1