[VL] ALTER TABLE ADD COLUMN with DEFAULT returns NULL for existing data instead of default value

### Backend

VL (Velox)

### Bug description

**Expected behavior**: When using `ALTER TABLE ADD COLUMN` with a DEFAULT value, existing rows should return the default value for the newly added column when queried (via EXISTS_DEFAULT lazy backfill mechanism).

**Actual behavior**: Existing rows return `NULL` for the newly added column instead of the configured default value.

This affects the EXISTS_DEFAULT lazy backfill mechanism introduced in Spark 3.4.0 (SPARK-38334). While newly inserted data (after ALTER) correctly uses default values, old data (inserted before ALTER) returns NULL.

### Reproduction Steps

```scala
// Test case from GlutenInsertSuite
testGluten("ALTER ADD COLUMN DEFAULT - old INT data backfill") {
withTable("t") {
sql("create table t(a int) using parquet")

// Insert OLD data BEFORE ALTER
sql("insert into t values(1)")
sql("insert into t values(2)")

// ALTER TABLE ADD COLUMN with DEFAULT
sql("alter table t add column (b int default 99)")

// Query old data
spark.table("t").orderBy("a").show()
}
}
```

**Expected Result**:
```
+---+---+
| a| b|
+---+---+
| 1| 99|
| 2| 99|
+---+---+
```

**Actual Result**:
```
+---+----+
| a| b|
+---+----+
| 1|null|
| 2|null|
+---+----+
```

### Test Failure Output

```
== Results ==
!== Correct Answer - 2 == == Spark Answer - 2 ==
!struct<> struct
![1,99] [1,null]
![2,99] [2,null]
```

All 5 test cases in GlutenInsertSuite fail with the same pattern:
1. `ALTER ADD COLUMN DEFAULT - old INT data backfill` - returns NULL instead of 99
2. `ALTER ADD COLUMN DEFAULT - old STRING data backfill` - returns NULL instead of 'unknown'
3. `ALTER ADD COLUMN DEFAULT - old BOOLEAN data backfill` - returns NULL instead of true
4. `ALTER ADD COLUMN DEFAULT - old DATE data backfill` - returns NULL instead of date '2023-01-01'
5. `ALTER ADD COLUMN DEFAULT - multiple types old data backfill` - returns NULL for all default columns

### Gluten version

main branch

### Spark version

Spark-3.4.x, Spark-3.5.x, Spark-4.0.x, Spark-4.1.x

---

*This issue was generated with AI assistance (Claude Code).*

Start with the five ALTER ADD COLUMN DEFAULT cases in GlutenInsertSuite and reproduce the mismatch between old and newly inserted rows. Trace the EXISTS_DEFAULT lazy backfill behavior through the Velox backend, comparing the failing old-data path with the working new-data path. Done means all listed types return their configured defaults for rows inserted before ALTER.

Written by the indexing model from the issue text.