fix(harness): stabilize OAuth callback tests

[tatoalo]

Problem

The harness OAuth callback tests use real loopback HTTP servers and can exceed Vitest's default 5-second timeout when CI is under load

[trunk-io]

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 1b5b140.}

[stamphog]

The change is a trivial test timeout bump in OAuth tests, but it matched the auth deny-list and was classified as T2-never, requiring human review. Get a review from a teammate before re-requesting.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(harness): stabilize OAuth callback t..." | Re-trigger Greptile}