Fix IbisSource IntegrityError on Snowflake with ibis >= 12 (filter before projecting categorical stats)

[cpsievert]

Problem

IbisSource.populate_column_stats() raises on Snowflake with
ibis-framework >= 12 when a table has 2+ low-cardinality text columns:

IntegrityError: Cannot add <NotNull ...> to filter, they belong to
another relation

The categorical-stats subqueries were built as:

table.select(...).filter(table[col].notnull())

The NotNull predicate belongs to the base table relation, but
.filter() is called on the projection returned by .select(). Ibis 12's
Filter.__init__ rejects predicates whose relations don't include the
filter's parent, and Snowflake's compile path constructs that invalid
Filter node directly.

Two details made this easy to miss:

• It requires 2+ qualifying columns — ibis.union() with one argument
  returns its input unchanged, so the union path that exposes the issue is
  never hit.
• DuckDB doesn't reproduce it — its salvage rewrite rebinds the
  predicate and pushes the filter below the projection, masking the bug.

Fix

Filter before projecting, so the predicate belongs to the relation being
filtered:

table.filter(table[col].notnull()).select(...).distinct()

The subquery construction is extracted into
IbisSource._categorical_values_subquery() so the invariant is directly
testable. No behavior change otherwise.

Tests

• Structural regression test: asserts the Filter node's parent is the
  base DatabaseTable (not a Project). This is the faithful repro —
  verified it fails on the old expression shape and passes on the new one.
  A behavioral DuckDB test alone cannot catch this bug.
• Behavioral test: populate_column_stats populates categories for 2+
  low-cardinality text columns and excludes nulls.

Notes

• Workaround for affected users until release: pass
  categorical_threshold=0 to QueryChat().
• The R package is unaffected — it builds raw SQL strings via DBI rather
  than ibis expression graphs.