feat: Implement partition_statistics API for SortMergeJoinExec

[kumarUjjawal]

Which issue does this PR close?

• Part of Implement partition_statistics API for more operators #15873.

Rationale for this change

• The goal is to replace the old statistics API with the new partition_statistics

What changes are included in this PR?

• implemented the partition_statistics API for SortMergeJoinExec
• Removed the special case that returned unknown statistics for specific partitions.
• Added test_partition_statistics to verify both aggregate and specific partition statistics work correctly

Are these changes tested?

• Addes new tests

Are there any user-facing changes?

[kumarUjjawal]

cc @xudong963

[comphead]

Thanks @kumarUjjawal I feel the test might be expanded

For aggregate statistics (partition = None): Should return meaningful statistics computed from both inputs

For partition-specific statistics (partition = Some(0)): Currently returns unknown statistics, but ideally should return statistics specific to that partition

Currently it just checks column num?

[kumarUjjawal]

Thanks @kumarUjjawal I feel the test might be expanded

For aggregate statistics (partition = None): Should return meaningful statistics computed from both inputs

For partition-specific statistics (partition = Some(0)): Currently returns unknown statistics, but ideally should return statistics specific to that partition

Currently it just checks column num?

Good point. Thank you for the feedback.