[Spark] Retain the last watermark of a source with no update in the batch

[Eliaaazzz]

GlobalWatermarkHolder.advance() rebuilt the global watermark map from only the sources that had a queued update for the completed batch and overwrote the stored map with the result. A source that reported no progress in that batch lost its last known watermark, and since SparkTimerInternals.forStreamFromSources skips ids that are missing from the map, the input watermark of every consuming stage jumped ahead of the idle source, firing timers early and dropping its later elements as late.

computeNewWatermarks now starts from the stored map and merges the per-source updates over it, so an idle source keeps its last watermark until it reports again. A source whose high watermark reached the end of time is done and holds nothing back, so it is not retained past the batch that completed it; without this, its finite synchronized processing time and low watermark would constrain consumers forever. Retained sources can disagree on the synchronized processing time, so SparkTimerInternals takes the slowest one instead of asserting equality. The stored map is also read once per advance() instead of once per updated source.

Two CreateStream test scripts (testFlattenedWithWatermarkHold, shouldTriggerProcessingTimeTimerWithSparseKey) now advance their watermarks to infinity like the other scripts. Their scripts previously stopped at a finite watermark and the tests only terminated because the dropped source no longer held the global watermark back; with retention that stalled watermark would legitimately hold it back until the test timeout.

testWatermarkRetainedForSourceWithoutUpdate fails on master with source 2 absent from the map after the second advance(); testCompletedSourceAgesOut pins the report-then-age-out behavior.

Fixes #39822.

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