fix(ui): correctly display token usage across sessions

[onematchfox]

When testing #1500, I struggled to visually demonstrate the affect of the duplicated session history (aka context bloat) when chatting with agents. The current implementation of "Usage" within the UI (introduced in #679) uses Math.max independently per field (total, prompt, completion) across all tasks in a session. This produces incoherent values where total does not necessarily match prompt + completion, since each field¹ (most notably prompt) can peak during differing tasks. And, it doesn't correctly update to reflect usage across multiple API calls in a multi-turn conversation. All we see is which API call has the greatest of each type of token usage.

This PR addresses those shortcomings by:

• Updating the existing token usage display to reflect session total across multiple turns
• Show per-call token usage tooltip on tool call cards and text messages

Footnotes

1. not 100% certain here but I think that fields other than input could also peak at different levels within cached prompt scenarios. ↩

[Copilot]

Pull request overview

This PR fixes incoherent token-usage reporting in the chat UI by switching from per-field Math.max aggregation to true per-turn/per-session accumulation, and by surfacing per-call usage on individual messages/tool cards.

Changes:

• Rename token usage fields to prompt/completion (from input/output) and update UI accordingly.
• Accumulate session token totals across multiple invocations/turns and persist pending turn stats across React re-renders.
• Add per-message/per-tool-call token usage tooltips and inject per-call stats into message metadata.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
ui/src/types/index.ts	Updates TokenStats shape to prompt/completion for clearer semantics.
ui/src/lib/messageHandlers.ts	Implements per-invocation tagging + session accumulation; injects tokenStats into message metadata.
ui/src/lib/tests/messageHandlers.test.ts	Adjusts and expands tests for message enrichment + summed stats across history/invocations.
ui/src/components/chat/ToolCallDisplay.tsx	Passes per-call tokenStats into tool call UI components.
ui/src/components/chat/TokenStatsTooltip.tsx	Adds a tooltip component to display per-call token usage.
ui/src/components/chat/TokenStats.tsx	Updates session usage display to prompt/completion and renames component.
ui/src/components/chat/ChatMessage.tsx	Adds per-message token usage tooltip next to feedback controls.
ui/src/components/chat/ChatInterface.tsx	Tracks session totals, preserves pending turn stats via ref, and memoizes handlers.
ui/src/components/chat/AgentCallDisplay.tsx	Displays per-call token usage tooltip on agent call cards.
ui/src/components/ToolDisplay.tsx	Displays per-call token usage tooltip on tool cards.

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