chore: Assert readiness directly instead of waiting on a timeout

[jsonbailey]

assert not ready.wait(0.1) cannot tell a correct refusal to signal from a slow machine, so these three tests read the flag directly instead.

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Note

Overview
Three datasource tests that expect initialization not to complete now assert not ready.is_set() instead of not ready.wait(0.1).

That removes a timing dependency: a short Event.wait could return false on a slow CI host even when readiness was correctly withheld, which made these negative assertions flaky. Behavior under test is unchanged—only how the tests observe the readiness flag.

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