What does this PR do?

Routes the prebuilt chat's remaining hard-coded strings through the labels config, so apps can translate them.

"Thought for N seconds" carries a number and plural rules, which a plain string label can't express. Its English default stays as it is. The reasoning header slot now also receives elapsedSeconds (React; Vue's header and layout slots), so an app can render its own text with correct plurals:

<CopilotChat
  messageView={{
    reasoningMessage: {
      header: ({ isStreaming, elapsedSeconds, ...props }) => (
        <CopilotChatReasoningMessage.Header
          {...props}
          isStreaming={isStreaming}
          label={isStreaming ? "Denkt nach…" : `${Math.round(elapsedSeconds)} Sekunden nachgedacht`}
        />
      ),
    },
  }}
/>

The change is additive: same English defaults, and no visual change unless an app sets the new labels. The reference pages for useCopilotChatConfiguration (React, Vue) and provideCopilotChatLabels list the new keys, and the reasoning-messages guide documents elapsedSeconds.

Not included:

• Angular's popup and sidebar close buttons keep their own names ("Close Copilot chat", "Close Copilot sidebar"). Angular's reasoning message has no header slot to pass elapsedSeconds to.
• The A2UI renderer's own strings ("Building interface", "Retrying generation…", "Filter options...") are left out, since the A2UI catalog is being reworked in feat: native generative UI (createGenerativeUI, tool and markdown transports, standard catalog, inspector pane) #7819 / feat: A2UI markdown mode (a2ui fences in chat, standard catalog) #7829.

Related PRs and Issues

• Closes 🚀 Feature Request: Translate the chat's remaining hard-coded strings ("Thinking…", screen-reader labels) through labels #7864
• Same pattern as fix(react-core): honor chatInputToolbarAddButtonLabel in the add menu tooltip #6771. Independent of fix(react-core,vue,angular): name the chat's icon-only buttons from labels #7865 (the icon-only button names): the two touch different keys and lines.

Testing

New tests. Each fails against main and passes here:

• React: CopilotChat.labels.test.tsx (7). Covers every string above, plus elapsedSeconds reaching a custom header (fake timers: 0, then 3 after three seconds).
• Vue: CopilotChat.labels.test.ts (6).
• Angular: copilot-chat-labels.spec.ts (2).

Checks:

• nx run-many -t check-types,test for react-core, vue and angular, run one after another: react-core 1905 tests, vue 1186, angular 543 (1 skipped), all passing, and type checks clean.
• nx run-many -t build,publint,attw for the same three: passing.
• oxlint and oxfmt --check on the changed files: no new warnings.

Browser check: a page built against @copilotkit/react-core packed from this branch, with German labels, in Chromium 149 via Playwright. Repro and capture script: https://github.com/Mordris/copilotkit-ui-repro

Checklist

• I have read the Contribution Guide
• If the PR changes or adds functionality, I have updated the relevant documentation
• If the PR contains a breaking change, the title has ! (for example feat(react-core)!: ...) and the description has a BREAKING CHANGE: line with migration steps. See VERSIONING.md (not a breaking change)
• "Allow edits by maintainers" is checked (lets us help iterate on your PR directly — faster turnaround for everyone)

Summary by CodeRabbit

• New Features
  • Chat labels can now be customized across Angular, React, and Vue for reasoning status, slash-command menus, attachment controls, modal buttons, and sidebar accessibility names.
  • Reasoning headers can access elapsed reasoning time to create localized labels.
  • Default labels are provided when custom labels are not set.
• Documentation
  • Updated configuration references and customization guides to cover the new labels and elapsed-time information.