examples: update gatekeeper auth client to use `onError`.

[thruflo]

This is what I requested in https://github.com/electric-sql/electric/pull/2018#issuecomment-2491085763

The client now uses the new onError to handle 401/403 and reconnect with a new auth token.

One observation: with the onError being defined before the shape is instantiated, I'm not sure what the best control flow is to handle a 405 must refetch. Certainly it's hard to handle in the onError callback. I guess this is what's being discussed in https://github.com/electric-sql/electric/issues/1997 etc.

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[KyleAMathews]

409 must-refetch responses never would reach onError — https://github.com/electric-sql/electric/blob/e9813969dccb679ab763773205f59a3fe7166bdc/packages/typescript-client/src/client.ts#L371-L377

[thruflo]

So the stream handles it and the shape gets the instruction in a control message?

Makes sense, cool.

[KyleAMathews]

So the stream handles it and the shape gets the instruction in a control message?

yeah exactly — the shape just resets its materialized view and recreates it — subscribers would never notice