[SPARK-52428] Fix show() to print table directly instead of double-wrapping

[rafafrdz]

Summary

• Fix `DataFrame::show()` to extract the formatted string from `ShowString` and print it directly
• Previously the result was wrapped in another Arrow table, producing a table-inside-a-table

Before:
```
+----------------+
| show_string |
+----------------+
| +-----+------+ |
| |score|status| |
| +-----+------+ |
| |20 |active| |
| +-----+------+ |
+----------------+
```

After:
```
+-----+------+
|score|status|
+-----+------+
|20 |active|
+-----+------+
```

Test plan

• `cargo build` passes
• `cargo fmt -- --check` passes