test: add UnknownEditFormat exception tests

tests/basic/test_coder.py

@@ -879,6 +879,27 @@ This command will print 'Hello, World!' to the console."""
             self.assertEqual(result, [])
             coder.commands.scraper.scrape.assert_not_called()
 
+    def test_unknown_edit_format_exception(self):
+        # Test the exception message format
+        invalid_format = "invalid_format"
+        valid_formats = ["diff", "whole", "map"]
+        exc = UnknownEditFormat(invalid_format, valid_formats)
+        expected_msg = f"Unknown edit format {invalid_format}. Valid formats are: {', '.join(valid_formats)}"
+        self.assertEqual(str(exc), expected_msg)
+
+    def test_unknown_edit_format_creation(self):
+        # Test that creating a Coder with invalid edit format raises the exception
+        io = InputOutput(yes=True)
+        invalid_format = "invalid_format"
+        
+        with self.assertRaises(UnknownEditFormat) as cm:
+            Coder.create(self.GPT35, invalid_format, io=io)
+        
+        exc = cm.exception
+        self.assertEqual(exc.edit_format, invalid_format)
+        self.assertIsInstance(exc.valid_formats, list)
+        self.assertTrue(len(exc.valid_formats) > 0)
+
     def test_coder_create_with_new_file_oserror(self):
         with GitTemporaryDirectory():
             io = InputOutput(yes=True)