[Douglasymlai]

Pull Request

Related Issue

Split out of #1958 (chat timeline presentation and controls) so it can be reviewed on its own. Builds on #2061, which added each Task's resolved model to the Run list.

Description

Problem
If you switch models partway through a Session, nothing in the chat shows which Tasks ran on which model. Reading back through a Session, you can't tell where the switch happened.

Cause
The chat had no per-Task model information. #2061 added the model each Task actually ran on to the Run list, but the chat never read it.

Fix

• A thin divider reading "Model changed to <model>" now appears above the first Task that ran on a different model from the Task before it.
• It works in the default chat view and in the event timeline (Normal and Detailed).
• The model comes from the Run list reads the Session already makes when it opens, so opening a Session costs no extra request. When a Task starts or finishes, one small read of the newest Task picks up its model, so the divider appears without reopening the Session.
• No divider is shown above the first Task, or when either Task's model is unknown (for example, a Task that stopped before it began running). A Task with an unknown model is skipped, so the next Task is compared with the last known model.
• Anything in the response that isn't a complete model identity is treated as unknown, never guessed.
• The label is translated in all 11 languages. It uses the shared Separator and meta text styles, and a long model name is truncated with its full text in a tooltip.

Testing Evidence (REQUIRED)

Automated:

• New tests cover reading models from the Run list (malformed values, Tasks from other Sessions, merging a newest-Task read), the divider rules (different model, same model, unknown on either side, first Task), placement in the default chat view including the read when a Task starts and finishes, and placement in the event timeline in both Normal and Detailed.
• npx vitest run --config vitest.config.ts test/unit/components/ChatBox test/unit/service test/unit/hooks: 1369 tests passed. One file, useInstallationSetup.test.ts, fails to load the same way on a clean main checkout, so it is unrelated to this change.
• npm run type-check, ESLint and Prettier on the changed files, npm run check:i18n, npm run check:design-tokens and git diff --check all pass.

Manual verification: not done yet. To check: open a Session, run a Task, switch the model, run another Task. The divider should appear above the second Task once it starts or finishes, in both the default chat view and the event timeline (VITE_CHATBOX_EVENT_BUS=true). Also check light and dark themes and a narrow window.

• I have included human-verified testing evidence in this PR.
• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).
• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline