fix(snowflake): allow expressions in window bounds

[ShawnStrasser]

Description of changes

Removes an overly restrictive check in the Snowflake SQL compiler that prevented expressions from being used in window bounds.

Snowflake added support for expressions in window bounds in August 2024 (release notes), but Ibis was still raising OperationNotDefinedError when trying to use interval expressions like ibis.interval(seconds=10).

Before: OperationNotDefinedError: Expressions in window bounds are not supported by Snowflake

After: Generates correct SQL like RANGE BETWEEN INTERVAL '10 SECOND' preceding AND INTERVAL '0 SECOND' following

The fix removes the restriction and lets the base class handle window boundary processing, matching the behavior of other SQL backends.

Issues closed

N/A - This addresses a compatibility issue with current Snowflake.

[cpcloud]

Confirmed this fixes expression bounds, and enables the corresponding test to now pass:

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