add hashcode/equals to WaitTest helper classes to avoid log error

[scwhittle]

There are lots of error logs showing up otherwise.
See https://github.com/apache/beam/actions/runs/13367967271/job/37335398780?pr=33979 for example:

    [direct-runner-worker] WARN org.apache.beam.sdk.util.MutationDetectors - Coder of type class org.apache.beam.sdk.schemas.SchemaCoder has a #structuralValue method which does not return true when the encoding of the elements is equal. Element org.apache.beam.sdk.transforms.WaitTest$WindowExpirationValue@aaa632c
    [direct-runner-worker] WARN org.apache.beam.sdk.util.MutationDetectors - Coder of type class org.apache.beam.sdk.schemas.SchemaCoder has a #structuralValue method which does not return true when the encoding of the elements is equal. Element org.apache.beam.sdk.transforms.WaitTest$WindowExpirationValue@aaa632c
    [direct-runner-worker] WARN org.apache.beam.sdk.util.MutationDetectors - Coder of type class org.apache.beam.sdk.schemas.SchemaCoder has a #structuralValue method which does not return true when the encoding of the elements is equal. Element org.apache.beam.sdk.transforms.WaitTest$WindowExpirationValue@aaa632c
    [direct-runner-worker] WARN org.apache.beam.sdk.util.MutationDetectors - Coder of type class org.apache.beam.sdk.schemas.SchemaCoder has a #structuralValue method which does not return true when the encoding of the elements is equal. Element org.apache.beam.sdk.transforms.WaitTest$WindowExpirationValue@aaa632c
    [direct-runner-worker] WARN org.apache.beam.sdk.util.MutationDetectors - Coder of type class org.apache.beam.sdk.schemas.SchemaCoder has a #structuralValue method which does not return true when the encoding of the elements is equal. Element org.apache.beam.sdk.transforms.WaitTest$WindowExpirationValue@6cff1a6a
    [direct-runner-worker] WARN org.apache.beam.sdk.util.MutationDetectors - Coder of type class org.apache.beam.sdk.schemas.SchemaCoder has a #structuralValue method which does not return true when the encoding of the elements is equal. Element org.apache.beam.sdk.transforms.WaitTest$WindowExpirationValue@6cff1a6a

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[scwhittle]

Run Java_PVR_Prism_Loopback PreCommit

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[m-trieu]

LGTM