feat: Cast string to timestamp_ntz (#4034)

* feat: cast string to timestamp_ntz

Author: parthchandra

docs/source/user-guide/latest/compatibility/expressions/cast.md

@@ -58,6 +58,14 @@ suffixes (e.g. `Europe/Moscow`), and the full Spark timestamp year range
 (-290308 to 294247). Note that `CAST(string AS DATE)` is only compatible for years between
 262143 BC and 262142 AD due to an underlying library limitation.
 
+## String to TimestampNTZ
+
+Comet's native `CAST(string AS TIMESTAMP_NTZ)` implementation matches Apache Spark's behavior.
+Unlike `CAST(string AS TIMESTAMP)`, this cast is timezone-independent: any timezone offset in
+the input string (e.g. `+08:00`, `Z`, `UTC`) is silently discarded, and the local date-time
+components are preserved as-is. Time-only strings (e.g. `T12:34:56`, `12:34`) produce `NULL`.
+The result is always a wall-clock timestamp with no timezone conversion or DST adjustment.
+
 ## Decimal with Negative Scale to String
 
 Casting a `DecimalType` with a negative scale to `StringType` is marked as incompatible when

native/spark-expr/src/conversion_funcs/cast.rs

@@ -28,7 +28,8 @@ use crate::conversion_funcs::numeric::{
 };
 use crate::conversion_funcs::string::{
     cast_string_to_date, cast_string_to_decimal, cast_string_to_float, cast_string_to_int,
-    cast_string_to_timestamp, is_df_cast_from_string_spark_compatible, spark_cast_utf8_to_boolean,
+    cast_string_to_timestamp, cast_string_to_timestamp_ntz,
+    is_df_cast_from_string_spark_compatible, spark_cast_utf8_to_boolean,
 };
 use crate::conversion_funcs::temporal::{
     cast_date_to_timestamp, is_df_cast_from_date_spark_compatible,
@@ -316,6 +317,9 @@ pub(crate) fn cast_array(
         (Null, _) => Ok(cast_with_options(&array, to_type, &native_cast_options)?),
         (Utf8, Boolean) => spark_cast_utf8_to_boolean::<i32>(&array, eval_mode),
         (LargeUtf8, Boolean) => spark_cast_utf8_to_boolean::<i64>(&array, eval_mode),
+        (Utf8, Timestamp(_, None)) => {
+            cast_string_to_timestamp_ntz(&array, eval_mode, true, cast_options.is_spark4_plus)
+        }
         (Utf8, Timestamp(_, _)) => cast_string_to_timestamp(
             &array,
             to_type,