Fix single-element list unwrapping in tool content

Single-element lists like [1] were being incorrectly unwrapped to "1"
in unstructured content while multi-element lists remained as lists.
This created inconsistent behavior where the structure was lost for
single items.

This fix ensures lists always preserve their structure in unstructured
content regardless of length, making behavior consistent and predictable.

Also removes pretty-printing from JSON serialization for more compact
output across tools, prompts, and resources.

Fixes #1064

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

Co-Authored-By: Claude <noreply@anthropic.com>

tests/resources/test_function_resources.py

@@ -67,7 +67,7 @@ class TestFunctionResource:
         )
         content = await resource.read()
         assert isinstance(content, str)
-        assert '"key": "value"' in content
+        assert '"key":"value"' in content
 
     async def test_error_handling(self):
         """Test error handling in FunctionResource."""
@@ -95,7 +95,7 @@ class TestFunctionResource:
             fn=lambda: MyModel(name="test"),
         )
         content = await resource.read()
-        assert content == '{\n  "name": "test"\n}'
+        assert content == '{"name":"test"}'
 
     async def test_custom_type_conversion(self):
         """Test handling of custom types."""