Pydantic v2.12 release (Python 3.14)

https://pydantic.dev/articles/pydantic-v2-12-release

• Support for Python 3.14
• New experimental MISSING sentinel
• Support for PEP 728 (TypedDict with extra_items)
• Preserve empty URL paths (url_preserve_empty_path)
• Control timestamp validation unit (val_temporal_unit)
• New exclude_if field option
• New ensure_ascii JSON serialization option
• Per-validation extra configuration
• Strict version check for pydantic-core
• JSON Schema improvements (regex for Decimal, custom titles, etc.)
• Only latest mypy version officially supported
• Slight validation performance improvement