Subtraction overflow in `max_distinct_count` when hash join has a pushed-down limit

[gabotechs]

Describe the bug

max_distinct_count in datafusion/physical-plan/src/joins/utils.rs panics with "attempt to subtract with overflow" in the Precision::Exact branch (line 725):

Precision::Exact(count) => {
    let count = count - stats.null_count.get_value().unwrap_or(&0); // <-- panic

This happens when num_rows (Exact) is smaller than null_count, which became possible after #20228, which added fetch support to HashJoinExec. When a limit is pushed down, HashJoinExec::partition_statistics() calls stats.with_fetch(self.fetch, 0, 1), which reduces num_rows to Exact(fetch_value) but does not reduce null_count in column statistics.

Example failing pipeline:
https://github.com/datafusion-contrib/datafusion-distributed/actions/runs/22798285744/job/66136064932?pr=366

To Reproduce

git clone https://github.com/datafusion-contrib/datafusion-distributed
cd datafusion-distributed
git checkout branch-53
cargo test --test tpcds_plans_test tests::test_tpcds_19 --all-features

Expected behavior

No substraction overflow

Additional context

No response

[KARTIK64-rgb]

Hi, I would like to work on this issue! I have analyzed the root cause ,the Precision::Exact branch in max_distinct_count uses a plain subtraction (count - null_count) which can overflow when null_count > count after a fetch/limit pushdown via HashJoinExec. The fix is to apply the same checked_sub pattern already used in the Inexact branch. Could you please assign this issue to me? Thanks!

[jonathanc-n]

@KARTIK64-rgb You can assign yourself by commenting take