test: add date_trunc DST regression test in non-UTC session timezone

spark/src/test/resources/sql-tests/expressions/datetime/trunc_timestamp_dst.sql

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+-- to you under the Apache License, Version 2.0 (the
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+--
+--   http://www.apache.org/licenses/LICENSE-2.0
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+-- under the License.
+
+-- Exercise Comet's native date_trunc on a timezone that observes DST.
+-- CometTruncTimestamp marks non-UTC as Incompatible, so enable allowIncompatible
+-- to force the native path and verify correctness against Spark.
+
+-- Config: spark.comet.expression.TruncTimestamp.allowIncompatible=true
+-- Config: spark.sql.session.timeZone=America/Los_Angeles
+
+statement
+CREATE TABLE test_trunc_dst(ts timestamp) USING parquet
+
+statement
+INSERT INTO test_trunc_dst VALUES
+  (timestamp('2024-11-03 06:30:00')),
+  (timestamp('2024-11-03 14:30:00')),
+  (timestamp('2024-03-10 05:30:00')),
+  (timestamp('2024-03-10 12:30:00')),
+  (timestamp('2024-11-15 12:00:00')),
+  (timestamp('2024-12-15 23:30:00')),
+  (timestamp('2024-07-15 10:00:00')),
+  (NULL)
+
+-- DAY truncation on a time after fall-back (PST) produces midnight of the same
+-- day, which is BEFORE the fall-back transition and should be in PDT.
+query
+SELECT ts, date_trunc('DAY', ts) FROM test_trunc_dst ORDER BY ts
+
+-- HOUR truncation crossing DST boundaries.
+query
+SELECT ts, date_trunc('HOUR', ts) FROM test_trunc_dst ORDER BY ts
+
+-- WEEK truncation can span DST transitions (the week of Nov 3 2024 starts Oct 28 PDT).
+query
+SELECT ts, date_trunc('WEEK', ts) FROM test_trunc_dst ORDER BY ts
+
+-- MONTH truncation: Nov 15 PST truncated to MONTH gives Nov 1, which is PDT.
+query
+SELECT ts, date_trunc('MONTH', ts) FROM test_trunc_dst ORDER BY ts
+
+-- QUARTER truncation: the motivating case from the PR (Dec PST -> Oct PDT).
+query
+SELECT ts, date_trunc('QUARTER', ts) FROM test_trunc_dst ORDER BY ts
+
+-- YEAR truncation: Dec 15 PST truncated to YEAR gives Jan 1, which is PST too, so no DST crossing.
+query
+SELECT ts, date_trunc('YEAR', ts) FROM test_trunc_dst ORDER BY ts