test: add sql-file test confirming fallback on parquet variant reads

[andygrove]

Which issue does this PR close?

Part of #1637

Rationale for this change

Comet does not support Spark 4.0's VARIANT data type, so scans exposing a VariantType column must fall back to Spark. There is no regression test pinning this behavior today. If a future change silently lets a VariantType column through the scan, we would only notice through runtime errors or wrong results. A targeted sql-file test locks in the fallback contract.

What changes are included in this PR?

• New file spark/src/test/resources/sql-tests/expressions/misc/variant.sql
• Gated with -- MinSparkVersion: 4.0 so it is skipped on Spark 3.4 / 3.5 via CometSqlFileTestSuite
• Creates parquet-backed tables with a VARIANT column (directly and nested inside STRUCT<v: VARIANT>), populated via parse_json
• Uses query expect_fallback(Unsupported v of type VariantType) on five queries covering projection, variant_get extraction, variant_get in a predicate, a COUNT(*) with a not-null filter, and a struct-containing-variant projection

How are these changes tested?

Ran the new test under the Spark 4.0 profile:

./mvnw test -Pspark-4.0 -Dsuites="org.apache.comet.CometSqlFileTestSuite variant" -Dtest=none

All five queries in the file fall back with the expected reason and the suite reports Tests: succeeded 1, failed 0.