fix: preserve duplicate GROUPING SETS rows

[xiedeyantu]

Which issue does this PR close?

• Closes #.

Rationale for this change

GROUPING SETS with duplicate grouping lists were incorrectly collapsed during execution. The internal grouping id only encoded the null mask, so repeated grouping sets shared the same key and were merged, which caused rows to be lost compared with PostgreSQL behavior.

I verified the expected result in PostgreSQL, where the same query returns 30 rows for the full emp example instead of being collapsed.

What changes are included in this PR?

• Preserve duplicate grouping sets by adding a per-occurrence ordinal into the internal grouping id used during execution.
• Keep GROUPING() semantics unchanged.
• Add regression coverage for the duplicate GROUPING SETS case in:
  • datafusion/core/tests/sql/aggregates/basic.rs
  • datafusion/sqllogictest/test_files/group_by.slt

Are these changes tested?

• cargo fmt --all
• cargo test -p datafusion duplicate_grouping_sets_are_preserved -- --nocapture
• PostgreSQL validation against the same query/result shape

Are there any user-facing changes?

• Yes. Queries that contain duplicate GROUPING SETS entries now return the correct duplicated result rows, matching PostgreSQL behavior.

[xiedeyantu]

Hi @alamb , there's another one here.