bug: `union` fails if string lengths do not match in 10.4.0

[rwhitten577]

What happened?

After upgrading existing Ibis code that uses Table.union to 10.4.0, I started receiving error messages about string lengths not matching, e.g.:

 Traceback (most recent call last):
    |   File "/Users/.../.venv/lib/python3.11/site-packages/ibis/expr/operations/relations.py", line 343, in __init__
    |     missing_from_left = right.schema - left.schema
    |                         ~~~~~~~~~~~~~^~~~~~~~~~~~~
    |   File "/Users/.../.venv/lib/python3.11/site-packages/ibis/common/collections.py", line 247, in __sub__
    |     common_keys = self._check_conflict(other)
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "/Users/.../.venv/lib/python3.11/site-packages/ibis/common/collections.py", line 212, in _check_conflict
    |     raise ConflictingValuesError(conflicts)
    | ibis.common.exceptions.ConflictingValuesError: Conflicting values for keys:
    |   `customer_id`: string(50) != string(4000)

In this scenario I am trying to union tables created via backend.sql(query_str) where each source table has a different length for the column in Snowflake, e.g.
query 1:

select customer_id from some_table1

query 2:

select customer_id from  some_table2

Is this a bug, or expected behavior after the implementation of #9382?

I can understand preference of the error to force explicit casting, but wondering if there is a way to keep the old upscaling behavior for the cases when it doesn't matter if the smaller types are kept. To work around this I'd have to always cast a few hundred columns to strings of a fixed length.

What version of ibis are you using?

10.4.0

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

Snowflake

Relevant log output

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