Add test for siftUp behavior in MessageHeap add method

Adds a focused unit test to verify that when a new message with the smallest 'when' value is added to the heap, it is correctly moved to the top.

This explicitly validates the behavior of the private `siftUp` method as it is used by the public `add` method, ensuring the heap property is maintained upon insertion.


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Original Change: ag/35088928 (Note tests are based on the original CL but may add coverage beyond the specific changes to improve overall code health)

Test: ATP tests passed http://go/forrest-run/L39500030017397747
Bug: 431235865
Flag: TEST_ONLY

Change-Id: Ia09ac28c64cc56face55adb96890e7626765e46f