feat(datafusion): show pushed-down limit in IcebergTableScan EXPLAIN output (#2360)

## Which issue does this PR close?

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- Closes #2359.

## What changes are included in this PR?

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Emit ` limit:[N]` in `IcebergTableScan`'s `DisplayAs` output when a
`LIMIT` is pushed down to the scan. When no limit is pushed down, the
output is unchanged.

- Before (unchanged):
`IcebergTableScan projection:[id,name] predicate:[...]`

- After (new, only when a limit reaches the scan):
`IcebergTableScan projection:[...] predicate:[] limit:[3]`

## Are these changes tested?

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Yes: new `EXPLAIN ... LIMIT 3` assertion in
`crates/sqllogictest/testdata/slts/df_test/basic_queries.slt`. Existing
snapshots are unchanged, which confirms the additive-only behavior.

Author: toutane

crates/integrations/datafusion/src/physical_plan/scan.rs

@@ -196,7 +196,11 @@ impl DisplayAs for IcebergTableScan {
             self.predicates
                 .clone()
                 .map_or(String::from(""), |p| format!("{p}"))
-        )
+        )?;
+        if let Some(limit) = self.limit {
+            write!(f, " limit:[{limit}]")?;
+        }
+        Ok(())
     }
 }
 

crates/sqllogictest/testdata/slts/df_test/basic_queries.slt

@@ -43,6 +43,18 @@ INSERT INTO default.default.query_test_table VALUES
 ----
 10
 
+# Verify EXPLAIN shows limit is pushed down to IcebergTableScan
+query TT
+EXPLAIN SELECT * FROM default.default.query_test_table LIMIT 3
+----
+logical_plan
+01)Limit: skip=0, fetch=3
+02)--TableScan: default.default.query_test_table projection=[id, name, score, category], fetch=3
+physical_plan
+01)GlobalLimitExec: skip=0, fetch=3
+02)--CooperativeExec
+03)----IcebergTableScan projection:[id,name,score,category] predicate:[] limit:[3]
+
 # Test SELECT * with ORDER BY and LIMIT
 query ITRT
 SELECT * FROM default.default.query_test_table ORDER BY id LIMIT 3