fix(xaa): address review nits from #1593 & use discoveryState#1721

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pcarleton wants to merge 3 commits intomainfrom

pcarleton/pcp-113-xaa-nits

Commits on Mar 20, 2026

fix(xaa): use OAuthTokens type, parseErrorResponse, and coerce expires_in
pcarleton
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refactor(xaa): consolidate save methods into OAuthDiscoveryState
pcarleton
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refactor(xaa): drop resourceUrl from OAuthDiscoveryState
pcarleton
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