Make the dashboard auth tests independent of host load #4331

Two tests added by #4301 time out in a full dashboard vitest run on a loaded host (load average about 170), and pass alone:

• src/data/auth/authenticatedEventSource.dom.test.ts "routes the app event stream through an authenticated fetch" (5030 ms)
• src/data/auth/credential.dom.test.ts "authenticates requests through the shared JSON transport" (7279 ms)

Taking 5 to 7 s suggests they wait on real timers, retries, or a reconnect delay rather than on the request itself.

Acceptance: find what each test waits on, make it deterministic (fake timers or an explicit resolution signal, not a longer timeout), keep it failing when the guarded behavior breaks (mutation check), and show both pass in a full vitest run under load.