bug: chained casts are incorrectly merged

[NickCrews]

What happened?

import ibis

ibis.memtable({"x": [1.2, 3.4]}).x.round().cast(int).cast(str).execute().to_list()
# gives
# ['1.0', '3.0']
# I expect
# ['1', '3']

# not a problem with literals?
ibis.literal(1.2).round().cast(int).cast(str).execute()
# '1'

What version of ibis are you using?

main

What backend(s) are you using, if any?

duckdb

Relevant log output

No response

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