fix: exclude *args and **kwargs from converted tool parameters

[betacatsling]

Summary

Fixes #735.

convert_function_to_tool turns every parameter in a function's signature into a required string property, including *args and **kwargs. For example, def f(x: int, *args, **kwargs) produces a tool schema with properties: {x, args, kwargs} and required: ['x', 'args', 'kwargs'] — the model is then told it must supply meaningless args/kwargs string arguments.

Variadic parameters cannot be expressed in the tool JSON schema, so this skips VAR_POSITIONAL and VAR_KEYWORD parameters when building the pydantic model used to generate the schema. Named parameters are unaffected.

Validation

• New regression test test_function_with_varargs in tests/test_utils.py asserts the generated parameters contain only x and required == ['x'].
• Before this change: fails — properties were {'x', 'args', 'kwargs'} and required included 'args'/'kwargs'.
• After: pytest tests/test_utils.py — 11 passed; full suite pytest tests/ — 98 passed.
• ruff check and ruff format --check pass on changed files.