bug: iceberg merge on read doesn't respect time travel

[chenzl25]

Describe the bug

If a query specifies a timestamp (the time after the flush statement before the delete statement) to time travel, we should use the timestamp corresponding snapshot to plan.

create table t (a int, b int, primary key (b, a)) with (commit_checkpoint_interval = 1) engine = iceberg;
CREATE_TABLE
dev=> insert into t values (1, 2);
INSERT 0 1

dev=> flush;

dev=> delete from t;

Error message/log

To Reproduce

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Expected behavior

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How did you deploy RisingWave?

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The version of RisingWave

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Additional context

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