fix: Address Copilot code review comments

- Removed empty TestDecodeJSONIndentation class (was adding unnecessary code)
- Fixed flawed assertion logic in test_json_indent_with_different_indent_sizes
- Changed return type from Any to Union[JsonValue, str] for better type safety
  (makes it clear the function returns either a Python value or a JSON string)
- Added Union to typing imports in decoder.py

All 19 spec-based json_indent tests passing ✓

Author: jreakin

src/toon_format/decoder.py

@@ -9,7 +9,7 @@
 """
 
 import json
-from typing import Any, Dict, List, Optional, Tuple
+from typing import Any, Dict, List, Optional, Tuple, Union
 
 from ._literal_utils import is_boolean_or_null_literal, is_numeric_literal
 from ._parsing_utils import (
@@ -229,7 +229,7 @@ def split_key_value(line: str) -> Tuple[str, str]:
     return (key, value)
 
 
-def decode(input_str: str, options: Optional[DecodeOptions] = None) -> Any:
+def decode(input_str: str, options: Optional[DecodeOptions] = None) -> Union[JsonValue, str]:
     """Decode a TOON-formatted string to a Python value.
 
     This function parses TOON format and returns the decoded data. By default,

tests/test_api.py

@@ -292,14 +292,8 @@ def test_roundtrip_with_length_marker(self):
         assert decoded == original
 
 
-class TestDecodeJSONIndentation:
-    """Test decode() JSON indentation feature (Issue #10).
-    
-    Comprehensive tests for the json_indent feature are in TestDecodeJSONIndentationWithSpecFixtures,
-    which validates against official TOON specification fixtures.
-    """
-
-    pass
+# Comprehensive tests for the json_indent feature are in TestDecodeJSONIndentationWithSpecFixtures,
+# which validates against official TOON specification fixtures from the TOON spec repository.
 
 
 def _get_sample_decode_fixtures() -> List[tuple]:
@@ -380,9 +374,8 @@ def test_json_indent_with_different_indent_sizes(
 
         # Different indent sizes should produce different strings (unless single line)
         if "\n" in result_2 and "\n" in result_4:
-            # Multi-line results should differ in formatting
-            # (indentation characters will be different)
-            assert result_2 != result_4 or result_2.count(" ") == result_4.count(" ")
+            # Multi-line results should differ in formatting and whitespace count
+            assert result_2 != result_4 and result_2.count(" ") != result_4.count(" ")
 
     def test_json_indent_consistency_with_plain_decode(self):
         """Verify that json_indent=None produces same data as plain decode."""