refactor(isthmus): make withUnquotedCasing return a copy, not mutate in place

withUnquotedCasing now follows the same copy-returning convention as
Calcite's own SqlParser.Config.with*() methods. The field is final and
set via the widest constructor (new 8-arg overload). All narrower
constructors default to Casing.TO_UPPER as before.

This closes the latent footgun where a caller assuming copy semantics
would silently mutate shared state, and eliminates the inconsistency
between live and snapshotted parser config reads in
SqlExpressionToSubstrait.

Author: nielspardon

isthmus/src/main/java/io/substrait/isthmus/ConverterProvider.java

@@ -55,11 +55,8 @@ public class ConverterProvider {
   /** The collection of Substrait extensions (functions and types) available for conversion. */
   protected final SimpleExtension.ExtensionCollection extensions;
 
-  /**
-   * The casing applied to unquoted SQL identifiers during parsing. Defaults to {@link
-   * Casing#TO_UPPER}.
-   */
-  protected Casing unquotedCasing = Casing.TO_UPPER;
+  /** The casing applied to unquoted SQL identifiers during parsing. */
+  protected final Casing unquotedCasing;
 
   /** Converter for Substrait scalar functions. */
   protected ScalarFunctionConverter scalarFunctionConverter;
@@ -107,7 +104,9 @@ public ConverterProvider(
         new ScalarFunctionConverter(extensions.scalarFunctions(), typeFactory),
         new AggregateFunctionConverter(extensions.aggregateFunctions(), typeFactory),
         new WindowFunctionConverter(extensions.windowFunctions(), typeFactory),
-        TypeConverter.DEFAULT);
+        TypeConverter.DEFAULT,
+        createDefaultExecutionBehavior(),
+        Casing.TO_UPPER);
   }
 
   /**
@@ -127,7 +126,15 @@ public ConverterProvider(
       AggregateFunctionConverter afc,
       WindowFunctionConverter wfc,
       TypeConverter tc) {
-    this(typeFactory, extensions, sfc, afc, wfc, tc, createDefaultExecutionBehavior());
+    this(
+        typeFactory,
+        extensions,
+        sfc,
+        afc,
+        wfc,
+        tc,
+        createDefaultExecutionBehavior(),
+        Casing.TO_UPPER);
   }
 
   /**
@@ -149,13 +156,39 @@ public ConverterProvider(
       WindowFunctionConverter wfc,
       TypeConverter tc,
       Plan.ExecutionBehavior executionBehavior) {
+    this(typeFactory, extensions, sfc, afc, wfc, tc, executionBehavior, Casing.TO_UPPER);
+  }
+
+  /**
+   * Creates a ConverterProvider with full customization including execution behavior and unquoted
+   * identifier casing.
+   *
+   * @param typeFactory the Calcite type factory to use
+   * @param extensions the Substrait extension collection to use
+   * @param sfc the scalar function converter to use
+   * @param afc the aggregate function converter to use
+   * @param wfc the window function converter to use
+   * @param tc the type converter to use
+   * @param executionBehavior the execution behavior to use for plans
+   * @param unquotedCasing the casing to apply to unquoted SQL identifiers during parsing
+   */
+  public ConverterProvider(
+      RelDataTypeFactory typeFactory,
+      SimpleExtension.ExtensionCollection extensions,
+      ScalarFunctionConverter sfc,
+      AggregateFunctionConverter afc,
+      WindowFunctionConverter wfc,
+      TypeConverter tc,
+      Plan.ExecutionBehavior executionBehavior,
+      Casing unquotedCasing) {
     this.typeFactory = typeFactory;
     this.extensions = extensions;
     this.scalarFunctionConverter = sfc;
     this.aggregateFunctionConverter = afc;
     this.windowFunctionConverter = wfc;
     this.typeConverter = tc;
     this.executionBehavior = executionBehavior;
+    this.unquotedCasing = unquotedCasing;
   }
 
   /**
@@ -181,18 +214,26 @@ public Casing getUnquotedCasing() {
   }
 
   /**
-   * Sets the casing applied to unquoted SQL identifiers during parsing and returns this instance
-   * for method chaining.
+   * Returns a new {@code ConverterProvider} identical to this one except with the given unquoted
+   * identifier casing. Follows the same copy-returning convention as Calcite's own {@code
+   * SqlParser.Config.withUnquotedCasing(Casing)}.
    *
    * <p>The default is {@link Casing#TO_UPPER}. Use {@link Casing#UNCHANGED} to preserve the
    * original casing of unquoted identifiers in SQL statements.
    *
    * @param unquotedCasing the casing to apply to unquoted identifiers
-   * @return this {@code ConverterProvider} instance
+   * @return a new {@code ConverterProvider} with the specified casing
    */
   public ConverterProvider withUnquotedCasing(Casing unquotedCasing) {
-    this.unquotedCasing = unquotedCasing;
-    return this;
+    return new ConverterProvider(
+        typeFactory,
+        extensions,
+        scalarFunctionConverter,
+        aggregateFunctionConverter,
+        windowFunctionConverter,
+        typeConverter,
+        executionBehavior,
+        unquotedCasing);
   }
 
   /**

isthmus/src/test/java/io/substrait/isthmus/UnquotedCasingTest.java

@@ -41,10 +41,13 @@ void withUnquotedCasingToLower() {
   }
 
   @Test
-  void withUnquotedCasingIsChainable() {
-    ConverterProvider provider = new ConverterProvider();
-    ConverterProvider returned = provider.withUnquotedCasing(Casing.UNCHANGED);
-    assertEquals(provider, returned);
+  void withUnquotedCasingReturnsCopy() {
+    ConverterProvider original = new ConverterProvider();
+    ConverterProvider copy = original.withUnquotedCasing(Casing.UNCHANGED);
+    // returns a distinct instance — original is not mutated
+    assert copy != original;
+    assertEquals(Casing.TO_UPPER, original.getUnquotedCasing());
+    assertEquals(Casing.UNCHANGED, copy.getUnquotedCasing());
   }
 
   /**