🪲 BUG-#247: Fix _serialize_context crash with non-Context list items

[FernandoCelmer]

Summary

• Fix _serialize_context crash when ctx.storage is a list of raw objects (dicts, strings, etc.) instead of Context objects
• Fix context_validator using truthy check instead of explicit None check
• Add support for tuple storage serialization
• Add _format_raw method for serializing non-Context values
• Configure pytest testpaths to exclude examples/

Test plan

• List of dicts serializes without error
• List of strings serializes without error
• List of Context objects still works
• Mixed list (Context + raw) serializes correctly
• Context without task_id in list uses index as key
• Tuple storage serializes correctly
• Empty list returns empty dict
• None context returns None
• Context with None storage returns None
• All 449 existing tests pass

Closes #247