Retain model cost evidence in completed Thinking cards

[witbrock]

Outcome

Completed Thinking cards retain the model identity, token counts, and estimated cost already present on the live request.

Cause and scope

The live response correctly populated llmUsageCostSummary, but createThinkingCardHistorySnapshot() omitted it while converting the active card into the inline completed card. Canonical history reload therefore worked while immediate completion lost the display. This PR adds only that omitted field and strengthens the existing active-to-retained-card test.

Validation

• npm test -- src/frontend/web/von_interface/static/js/test/chatTab.test.js --runInBand — 251 passed
• npm run lint:frontend:static -- src/frontend/web/von_interface/static/js/chatTab.js src/frontend/web/von_interface/static/js/test/chatTab.test.js — passed
• git diff --check — passed

Jira: JVNAUTOSCI-2624

[Copilot]

Pull request overview

This PR fixes a UI regression where completed (retained) Thinking cards lost the model usage/cost evidence that was already present on the live request, by carrying llmUsageCostSummary into the history snapshot used for the inline retained card.

Changes:

• Preserve llmUsageCostSummary when converting an active Thinking card into its retained (completed) snapshot.
• Strengthen the Thinking-card retention accessibility test to assert model usage details are still rendered after completion.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/frontend/web/von_interface/static/js/chatTab.js	Adds llmUsageCostSummary to the retained Thinking card snapshot so immediate completion retains model usage/cost display.
src/frontend/web/von_interface/static/js/test/chatTab.test.js	Extends the retained Thinking card test to verify “Model usage” content (model identity, tokens, and estimated cost) is present after completion.