fix: make ChatRequest serializer None-safe for tools without parameters or function

[betacatsling]

Summary

Fixes #733.

ChatRequest.serialize_model assumes every serialized tool has a non-None function containing a non-None parameters mapping. When a Tool is created without function, or a Tool.Function is created without parameters, model_dump() (and model_dump_json()) without exclude_none=True raises PydanticSerializationError: TypeError: argument of type 'NoneType' is not iterable because the serializer evaluates 'parameters' in None / 'defs' in None.

This makes the serializer tolerate missing function/parameters so request objects can be dumped freely (e.g. for logging or debugging) regardless of how the tools were constructed. The defs → $defs rename behavior is unchanged when parameters is present.

Validation

• New regression tests in tests/test_type_serialization.py:
  • test_chat_request_serialization_tool_without_parameters
  • test_chat_request_serialization_tool_without_function
  • test_chat_request_serialization_tool_defs_alias
• Before this change: the first two tests fail with PydanticSerializationError: Error calling function 'serialize_model': TypeError: argument of type 'NoneType' is not iterable.
• After: pytest tests/test_type_serialization.py — 17 passed; full suite pytest tests/ — 100 passed.
• ruff check and ruff format --check pass on changed files.