Widget: setUser({ token }) should take a token function, like userToken #482

Follow-up to #476. userToken in the widget config can now be a function the widget calls for a fresh token each time it asks for a session (UserToken in packages/widget/src/api/client.ts:6). ChatCops.setUser({ token }) still takes only a string (packages/widget/src/widget.ts:501, and the same signature in packages/widget/src/index.ts and ChatCopsLoader in packages/widget/src/loader/loader.ts). So a site that signs the visitor in after the page loads, which is what setUser is for, can't hand over a function.

What to do

• Widen token in setUser to UserToken in all three places, and pass it through to client.setUserToken as it is.
• setUser compares the new token with the old one to decide whether the visitor changed and the chat should be cleared (widget.ts:502-504). Two different functions are always different, so define what counts as the same visitor: the simplest rule is that passing the same function again doesn't clear the chat, and anything else does. Document it.

Acceptance criteria

• Unit tests: setUser with a function, the function is called when a session is requested, the same function again doesn't clear the chat, and a different token or null does
• Type tests in packages/widget/tests/api.test-d.ts for both builds
• website/src/content/docs/widget/api.mdx (the setUser section) says it takes a function
• A patch changeset for @chatcops/widget; pnpm test:coverage, pnpm lint and pnpm size pass

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