refactor: Spark ascii signature away from user_defined (#19513)

## Which issue does this PR close?

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- Part of #12725 

## Rationale for this change

As per the goal stated that we should avoid using the `user_defined` in
useful places.

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## What changes are included in this PR?

- Refactored Spark `ascii` to not use `user_defined`. 
- Checked the Spark code to make sure it follows the same convention

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## Are these changes tested?

- This is a refactoring so no new tests were added but all previous test
pass.

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## Are there any user-facing changes?

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Author: kumarUjjawal

datafusion/spark/src/function/string/ascii.rs

@@ -17,8 +17,12 @@
 
 use arrow::datatypes::DataType;
 use datafusion_common::Result;
+use datafusion_common::types::{NativeType, logical_string};
 use datafusion_expr::ColumnarValue;
-use datafusion_expr::{ScalarFunctionArgs, ScalarUDFImpl, Signature, Volatility};
+use datafusion_expr::{
+    Coercion, ScalarFunctionArgs, ScalarUDFImpl, Signature, TypeSignatureClass,
+    Volatility,
+};
 use datafusion_functions::string::ascii::ascii;
 use datafusion_functions::utils::make_scalar_function;
 use std::any::Any;
@@ -42,8 +46,17 @@ impl Default for SparkAscii {
 
 impl SparkAscii {
     pub fn new() -> Self {
+        // Spark's ascii uses ImplicitCastInputTypes with StringType,
+        // which allows numeric types to be implicitly cast to String.
+        // See: https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala
+        let string_coercion = Coercion::new_implicit(
+            TypeSignatureClass::Native(logical_string()),
+            vec![TypeSignatureClass::Numeric],
+            NativeType::String,
+        );
+
         Self {
-            signature: Signature::user_defined(Volatility::Immutable),
+            signature: Signature::coercible(vec![string_coercion], Volatility::Immutable),
         }
     }
 }
@@ -68,8 +81,4 @@ impl ScalarUDFImpl for SparkAscii {
     fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
         make_scalar_function(ascii, vec![])(&args.args)
     }
-
-    fn coerce_types(&self, _arg_types: &[DataType]) -> Result<Vec<DataType>> {
-        Ok(vec![DataType::Utf8])
-    }
 }