Test: Session-level token usage data

[cwebster-99]

Refs: #315283

• any OS @chrmarti
• any OS @kycutler

Complexity: 3

Authors: @lramos15

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VS Code now surfaces session-level token usage, similar to the Copilot CLI's /usage. This supplements the existing per-turn credit consumption and context-window indicators to give a fuller picture of what a chat session has consumed.

Task

Validate that session-level token usage is reported and is accurate.

• Start a new agent/chat session and run several turns.
• Open the session token usage view and confirm:
  • Total session token consumption is shown and increases as turns are added
  • The session-level numbers are consistent with the per-turn credit / context-window info already shown
• Confirm the data resets / starts fresh for a new session.
• Try across model configurations (different context sizes / reasoning efforts) and verify the usage view still behaves correctly.

[chrmarti]

The usage indicator is missing in this session:

Is this a limitation of the model, Auto mode or a bug?

[chrmarti]

It's now showing after running a turn with Opus 4.8. It seems to show the total credits of the session, so I guess we're just not showing the context window indicator for Auto because that can switch models mid-session? Will open a bug for the missing total credits display with Auto.