feat(stats): cap NDV at row count in statistics estimation

[asolimando]

Which issue does this PR close?

• Part of EPIC: Making use of NDVs (number of distinct values) in DataFusion #20766

Rationale for this change

After a filter reduces a table from 100 to 10 rows, or a LIMIT 10 caps the output, the NDV (e.g. 80) should not exceed the new row count. Without capping, join cardinality estimation uses an inflated denominator, leading to inaccurate estimates.

What changes are included in this PR?

Cap distinct_count at num_rows in three places to prevent NDV from exceeding the actual row count:

• max_distinct_count in join cardinality estimation (joins/utils.rs)
• collect_new_statistics in filter output statistics (filter.rs)
• Statistics::with_fetch (stats.rs), which covers GlobalLimitExec, LocalLimitExec, SortExec (with fetch), CoalescePartitionsExec (with fetch), and CoalesceBatchesExec (with fetch)

Note: NDV capping for AggregateExec is covered separately in #20926.

Are these changes tested?

• test_filter_statistics_ndv_capped_at_row_count - verifies NDV capped at filtered row count
• 2 new join cardinality test cases - NDV > rows on both/one side
• Updated test_join_cardinality expected values for capped NDV
• test_with_fetch_caps_ndv_at_row_count - verifies NDV capped after LIMIT
• test_with_fetch_ndv_below_row_count_unchanged - verifies NDV untouched when already below row count
• All existing with_fetch tests pass

Are there any user-facing changes?

No public API changes. Only internal statistics estimation is affected.

Disclaimer: I used AI to assist in the code generation, I have manually reviewed the output and it matches my intention and understanding.

[asolimando]

cc: @jonathanc-n