Prism: Fix event-time timer firing earlier than the time set.

[shunping]

Fixes#35090.

Running the same test code from #35090 (comment), we no longer see timer callback being invoked.

Furthermore, if we change the test code to set firing timestamp of a timer to be the timestamp of first element + 2 (rather than 10 in the original test), we will see the timer callback is actually called.

set timer to Timestamp(1748577640.693000)
timer callback start (timestamp=Timestamp(1748577640.693000))
timer callback elements: [1748577638.693265, 1748577639.693265, 1748577640.693265]
timer callback end
batch process start
1748577638.693265
1748577639.693265
1748577640.693265
1748577641.693265
1748577642.693265
batch process end
clear timer
set timer to Timestamp(1748577645.693000)
timer callback start (timestamp=Timestamp(1748577645.693000))
timer callback elements: [1748577643.693265, 1748577644.693265, 1748577645.693265]
timer callback end
batch process start
1748577643.693265
1748577644.693265
1748577645.693265
1748577646.693265
1748577647.693265
batch process end
clear timer
set timer to Timestamp(1748577650.693000)
timer callback start (timestamp=Timestamp(1748577650.693000))
timer callback elements: [1748577648.693265, 1748577649.693265]
timer callback end
batch process start
1748577648.693265
1748577649.693265
1748577650.693265
1748577651.693265
1748577652.693265
batch process end
clear timer
set timer to Timestamp(1748577655.693000)
timer callback start (timestamp=Timestamp(1748577655.693000))
timer callback elements: [1748577653.693265, 1748577654.693265, 1748577655.693265]
timer callback end
batch process start
1748577653.693265
1748577654.693265
1748577655.693265
1748577656.693265
1748577657.693265
batch process end
...

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