Fix flaky test: TestTooManyCancelRequests/CancelAllWorkflowsAtOnce

[spkane31]

What changed?

Fixed race condition in TestClientMiscTestSuite/TestTooManyCancelRequests/CancelAllWorkflowsAtOnce by reordering test operations to wait for workflow completion before checking history.

Why?

The test was failing intermittently in CI with:

Error: Not equal: 
    expected: 4
    actual  : 2 (or 3)
Messages: Length of expected(4) and actual(2) histories is not equal

The test was checking workflow history immediately after starting the workflow, before ensuring the workflow task had been executed by a worker. This created a race condition where the test could check history before all events were persisted (WorkflowTaskStarted and WorkflowTaskFailed events).

The fix reorders operations to call run.Get() (which waits for workflow completion) before WaitForHistoryEvents(). This ensures proper synchronization and that all history events are persisted before verification.

This follows the same pattern used in TestTooManyPendingSignals in the same file.

How did you test it?

• built
• run locally and tested manually
• covered by existing tests