Map Throwable.getMessage() to Exception.Message (#165) (#174)

Fixes #165.

Java's `e.getMessage()` converted to `e.GetMessage()`, which doesn't
compile against C#'s `Exception`.

`TypeHelper.TryTransformMethodCall` already had a
`ReplaceMethodByProperty` helper backing `length`→`Length` and
`size`→`Count` — that's exactly the method-call-to-property-access
handling the issue noted was needed, so `getMessage`→`Message` slots in
there rather than into the name-only `ReplaceCommonMethodNames` table.

The case is guarded on zero arguments, matching the surrounding cases,
so an unrelated `obj.getMessage(i)` still converts to a normal method
call.

## Tests

- Two cases in `ConvertExpressionTests` (`e.getMessage()` → `e.Message`,
and the arity-guard negative case).
- A new `Resources/ExceptionGetMessage.java` fixture wired into
`FullIntegrationTests`, so the generated C# is compiled and run: it
throws an `IllegalArgumentException`, catches it, and asserts the
printed message.

Full suite passes at 363 tests.

## Note

Java's `Throwable` has sibling accessors with the same problem
(`getLocalizedMessage()`, `getCause()`). This PR stays scoped to
`getMessage()` as the issue described; happy to follow up on the others.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: paulirwin

JavaToCSharp.Tests/ConvertExpressionTests.cs

@@ -12,6 +12,8 @@ public class ConvertExpressionTests
     [InlineData("lst.set(i, value)", "lst[i] = value")]
     [InlineData("str.length()", "str.Length")]
     [InlineData("arr.length", "arr.Length")]
+    [InlineData("e.getMessage()", "e.Message")]
+    [InlineData("obj.getMessage(i)", "obj.GetMessage(i)")]
 
     //Conversion not done if param number does not match the required
     [InlineData("obj.size(i)", "obj.Size(i)")]

JavaToCSharp.Tests/IntegrationTests.cs

@@ -88,6 +88,7 @@ public void GeneralUnsuccessfulConversionTest(string filePath)
     [InlineData("Resources/InstanceInitializers.java")]
     [InlineData("Resources/StaticImports.java")]
     [InlineData("Resources/LabeledBreakContinue.java")]
+    [InlineData("Resources/ExceptionGetMessage.java")]
     public void FullIntegrationTests(string filePath, bool allowWarnings = false)
         => RunFullIntegrationTest(filePath, allowWarnings);
 

JavaToCSharp.Tests/Resources/ExceptionGetMessage.java

@@ -0,0 +1,13 @@
+/// Expect:
+/// - output: "caught: boom\n"
+package example;
+
+public class Program {
+    public static void main(String[] args) {
+        try {
+            throw new IllegalArgumentException("boom");
+        } catch (IllegalArgumentException e) {
+            System.out.println("caught: " + e.getMessage());
+        }
+    }
+}

JavaToCSharp/TypeHelper.cs

@@ -273,6 +273,13 @@ public static bool TryTransformMethodCall(ConversionContext context, MethodCallE
                     return true;
                 }
 
+                case "getMessage" when args.size() == 0:
+                {
+                    var scopeSyntaxMessage = ExpressionVisitor.VisitExpression(context, scope);
+                    transformedSyntax = ReplaceMethodByProperty(scopeSyntaxMessage, "Message");
+                    return true;
+                }
+
                 case "get" when args.size() == 1:
                 {
                     var scopeSyntaxGet = ExpressionVisitor.VisitExpression(context, scope);